Semiconductor device and method for manufacturing the same

ABSTRACT

An object is to provide a thin film transistor having favorable electric characteristics and a semiconductor device including the thin film transistor as a switching element. The thin film transistor includes a gate electrode formed over an insulating surface, a gate insulating film over the gate electrode, an oxide semiconductor film which overlaps with the gate electrode over the gate insulating film and which includes a layer where the concentration of one or a plurality of metals contained in the oxide semiconductor is higher than that in other regions, a pair of metal oxide films formed over the oxide semiconductor film and in contact with the layer, and a source electrode and a drain electrode in contact with the metal oxide films. The metal oxide films are formed by oxidation of a metal contained in the source electrode and the drain electrode.

TECHNICAL FIELD

The present invention relates to a thin film transistor including anoxide semiconductor, a semiconductor device including the thin filmtransistor, and a method for manufacturing the semiconductor device.

BACKGROUND ART

A thin film transistor including a semiconductor film formed over aninsulating surface is an essential semiconductor element for asemiconductor device. Since there is limitation on manufacture of thinfilm transistors in terms of allowable temperature limit of a substrate,a transistor mainly used for a semiconductor display device is a thinfilm transistor including amorphous silicon that can be deposited atrelatively low temperature, polysilicon that can be obtained bycrystallization with use of laser light or a catalytic element, or thelike in an active layer.

In recent years, a metal oxide having semiconductor characteristicswhich is referred to as an oxide semiconductor has attracted attentionas a novel semiconductor material which has both high mobility, which isa characteristic of polysilicon, and uniform element characteristics,which is a characteristic of amorphous silicon. The metal oxide is usedfor various applications; for example, indium oxide is a well-knownmetal oxide and used as a material of a transparent electrode includedin a liquid crystal display device or the like. Examples of such metaloxides having semiconductor characteristics include tungsten oxide, tinoxide, indium oxide, zinc oxide, and the like. A thin film transistorincluding such a metal oxide having semiconductor characteristics in achannel formation region has been known (Patent Documents 1 and 2).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055

DISCLOSURE OF INVENTION

An object of one embodiment of the present invention disclosed is toprovide a thin film transistor having favorable electric characteristicsand a semiconductor device including the thin film transistor as aswitching element.

The inventors found that a region which is the closest to a sourceelectrode and a drain electrode in an In—Ga—Zn—O-based oxidesemiconductor film includes composite layers where the concentration ofa metal is higher than that in other regions (metal-rich layers) in athin film transistor including the In—Ga—Zn—O-based oxide semiconductorfilm as an active layer of the thin film transistor. The inventors alsofound that metal oxide films are formed between the source electrode andthe composite layer, and between the drain electrode and the compositelayer.

FIG. 2 shows a photograph of a cross section of a thin film transistorwith a channel-etched structure in which the In—Ga—Zn—O-based oxidesemiconductor film is used as an active layer of the thin filmtransistor. The photograph is taken with a high resolution transmissionelectron microscope (TEM: “H9000-NAR” manufactured by Hitachi, Ltd.).Both FIGS. 3A and 3B show a high-magnification photograph(four-million-fold magnification) of the interface between an oxidesemiconductor film and a titanium film which is in contact with the topof the oxide semiconductor film, by using the same sample as that forthe photograph in FIG. 2. Both of the photographs are taken with ascanning transmission electron microscope (STEM: “HD-2700” manufacturedby Hitachi, Ltd.) at an accelerating voltage of 200 kV.

A photograph at Point A in FIG. 2 corresponds to FIG. 3A, and aphotograph at Point B in FIG. 2 corresponds to FIG. 3B. Specifically,FIG. 3A is a photograph of the interface between the oxide semiconductorfilm and the titanium film, which is in contact with the top of theoxide semiconductor film, at a position where the oxide semiconductorfilm overlaps with a gate electrode. As can be seen from FIG. 3A, thereis an interface layer containing titanium oxide (TiOx) between thetitanium (Ti) film and the In—Ga—Zn—O-based oxide semiconductor film(IGZO). In addition, in the In—Ga—Zn—O-based oxide semiconductor film(IGZO), a region which is the closest to the interface layer containingtitanium oxide (TiOx) includes an indium crystal, which can be seen as agrid shape. The layer containing indium that can be seen as a grid shapecorresponds to a composite layer where the concentration of indium ishigher than that in other regions (an In-rich layer).

In a similar manner, FIG. 3B is a photograph of the interface betweenthe oxide semiconductor film and the titanium film, which is in contactwith the top of the oxide semiconductor film, at a position where theoxide semiconductor film does not overlap with the gate electrode. In amanner similar to FIG. 3A, as can be seen from FIG. 3B, there is aninterface layer containing titanium oxide (TiOx) between the titanium(Ti) film and the In—Ga—Zn—O-based oxide semiconductor film (IGZO). Inaddition, in the In—Ga—Zn—O-based oxide semiconductor film (IGZO), aregion which is the closest to the interface layer containing titaniumoxide (TiOx) includes an In-rich layer.

The inventors thought that the titanium oxide is formed in the followingmanner: oxygen in the oxide semiconductor film is taken out by titaniumin the vicinity of the interface between the oxide semiconductor filmand the titanium film; the concentration of In is increased in a regionof the oxide semiconductor film which is close to the titanium film; andthe oxygen which is taken out is reacted with titanium.

Because a region which is the closest to a source electrode and a drainelectrode in an In—Ga—Zn—O-based oxide semiconductor film includeslayers where the concentration of one or a plurality of indium, gallium,and zinc is higher than that in other regions (metal-rich layers), themetal-rich layers in the oxide semiconductor film have low resistance.In addition, the titanium oxide films (TiOx) formed between the sourceelectrode and the oxide semiconductor film and between the drainelectrode and the oxide semiconductor film have n-type conductivity.Therefore, with the above structure, contact resistance between thesource electrode and the oxide semiconductor film and between the drainelectrode and the oxide semiconductor film is reduced, and the amount ofon-current and field effect mobility of the TFT can be increased.

It is possible to use as the oxide semiconductor, a four-component metaloxide such as an In—Sn—Ga—Zn—O-based oxide semiconductor, athree-component metal oxide such as an In—Ga—Zn—O-based oxidesemiconductor, an In—Sn—Zn—O-based oxide semiconductor, anIn—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxidesemiconductor, an Al—Ga—Zn—O-based oxide semiconductor, and aSn—Al—Zn—O-based oxide semiconductor, or a two-component metal oxidesuch as an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxidesemiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-basedoxide semiconductor, a Sn—Mg—O-based oxide semiconductor, anIn—Mg—O-based oxide semiconductor, an In—Ga—O-based oxide semiconductor,an In—O-based oxide semiconductor, a Sn—O-based oxide semiconductor, anda Zn—O-based oxide semiconductor. Note that in this specification, forexample, an In—Sn—Ga—Zn—O-based oxide semiconductor means a metal oxideincluding indium (In), tin (Sn), gallium (Ga), and zinc (Zn), and thereis no particular limitation on the stoichiometric proportion. The aboveoxide semiconductor may contain silicon.

Moreover, oxide semiconductors can be represented by the chemicalformula, InMO₃(ZnO)_(m) (m>0). Here, M represents one or more metalelements selected from Ga, Al, Mn, and Co.

A driver circuit and a pixel portion can be formed over one substrate byusing a thin film transistor which is one embodiment of the presentinvention, and a semiconductor display device can be manufactured byusing a display element such as an EL element, a liquid crystal element,or an electrophoretic element.

Since a thin film transistor is easily broken due to static electricityor the like, a protective circuit for protecting the thin filmtransistor for the pixel portion is preferably provided over the samesubstrate for a gate line or a source line. The protective circuit ispreferably formed using a nonlinear element in which an oxidesemiconductor film is used.

The thin film transistor which is one embodiment of the presentinvention may be a bottom-gate thin film transistor with achannel-etched structure, or may be a bottom-gate thin film transistorwith a channel-protective structure. Alternatively, the thin filmtransistor may be a bottom-contact thin film transistor.

The bottom-gate transistor includes a gate electrode formed over aninsulating surface, a gate insulating film over the gate electrode, anoxide semiconductor film which overlaps with the gate electrode over thegate insulating film and which includes composite layers where theconcentration of one or a plurality of metals contained in the oxidesemiconductor is higher than that in other regions, a pair of metaloxide films formed over the oxide semiconductor film and in contact withthe composite layers, and a source electrode and a drain electrode whichare in contact with the metal oxide films. The metal oxide films areformed by oxidation of a metal contained in the source electrode and thedrain electrode.

The bottom-contact transistor includes a gate electrode formed over aninsulating surface, a gate insulating film over the gate electrode, asource electrode and a drain electrode over the gate insulating film,metal oxide films in contact with the source electrode and the drainelectrode, and an oxide semiconductor film which overlaps with the gateelectrode and which includes composite layers where the concentration ofone or a plurality of metals contained in the oxide semiconductor ishigher than that in other regions. The composite layers are in contactwith the metal oxide films. The metal oxide films are formed byoxidation of a metal contained in the source electrode and the drainelectrode.

Because a region which is the closest to a source electrode and thedrain electrode in an oxide semiconductor film includes composite layerswhere the concentration of a metal is higher than that in other regions,and metal oxide films having n-type conductivity are formed between thesource electrode and the oxide semiconductor film and between the drainelectrode and the oxide semiconductor film, contact resistance betweenthe source electrode and the oxide semiconductor film and between thedrain electrode and the oxide semiconductor film is reduced, and theamount of on-current and field effect mobility of a TFT can beincreased.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1C illustrate cross-sectional views of a transistor, andFIG. 1B illustrates a top view thereof.

FIG. 2 shows a cross-sectional TEM photograph of a thin film transistor.

FIGS. 3A and 3B show cross-sectional TEM photographs in the vicinity ofthe interface between an oxide semiconductor film and a source electrodeor between the oxide semiconductor film and a drain electrode in a thinfilm transistor.

FIGS. 4A to 4C illustrate crystal structures of metals and oxygen inIGZO.

FIGS. 5A and 5B illustrate structural models of metal atoms and oxygenatoms in the vicinity of the interface between a tungsten film and anoxide semiconductor film.

FIGS. 6A and 6B illustrate structural models of metal atoms and oxygenatoms in the vicinity of the interface between a molybdenum film and anoxide semiconductor film.

FIGS. 7A and 7B illustrate structural models of metal atoms and oxygenatoms in the vicinity of the interface between a titanium film and anoxide semiconductor film.

FIG. 8 illustrates a crystal structure of titanium dioxide having arutile structure.

FIG. 9 shows a state density of titanium dioxide having a rutilestructure.

FIG. 10 shows a state density of titanium dioxide in anoxygen-deficiency state.

FIG. 11 shows a state density of a titanium monoxide.

FIGS. 12A and 12C illustrate cross-sectional views of a transistor, andFIG. 12B illustrates a top view thereof.

FIGS. 13A and 13C illustrate cross-sectional views of a transistor, andFIG. 13B illustrates a top view thereof.

FIGS. 14A and 14B respectively illustrate a top view and across-sectional view of an electronic paper.

FIGS. 15A and 15B illustrate block diagrams of semiconductor displaydevices.

FIGS. 16A and 16B illustrate configuration of a signal line drivercircuit and a timing chart thereof.

FIGS. 17A and 17B are circuit diagrams showing a structure of a shiftregister.

FIGS. 18A and 18B respectively show a circuit diagram and a timing chartof operation of a shift register.

FIGS. 19A to 19C show a method for manufacturing a semiconductor device.

FIGS. 20A to 20C show the method for manufacturing a semiconductordevice.

FIGS. 21A and 21B show the method for manufacturing a semiconductordevice.

FIG. 22 shows the method for manufacturing a semiconductor device.

FIG. 23 shows the method for manufacturing a semiconductor device.

FIG. 24 shows the method for manufacturing a semiconductor device.

FIG. 25 illustrates a cross-sectional view of a liquid crystal displaydevice.

FIGS. 26A to 26C illustrate cross-sectional views of light-emittingdevices.

FIG. 27 illustrates a structure of a liquid crystal display devicemodule.

FIGS. 28A to 28E illustrate electronic devices each using asemiconductor display device.

FIG. 29 illustrates a band diagram of an embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the following description and it is easilyunderstood by those skilled in the art that modes and details can bevariously changed without departing from the scope and spirit of thepresent invention. Therefore, the invention should not be construed asbeing limited to the description of the embodiments below.

The present invention can be applied to manufacture of any kind ofsemiconductor devices including microprocessors, integrated circuitssuch as image processing circuits, RF tags, and semiconductor displaydevices. The semiconductor display devices include the following in itscategory: liquid crystal display devices, light-emitting devices inwhich a light-emitting element typified by an organic light-emittingelement (OLED) is provided for each pixel, electronic papers, digitalmicromirror devices (DMDs), plasma display panels (PDPs), field emissiondisplays (FEDs), and other semiconductor display devices in which acircuit element using a semiconductor film is included in a drivercircuit.

Note that the semiconductor display devices include a panel in which adisplay element is sealed, and a module in which an IC and the likeincluding a controller are mounted on the panel. The present inventionfurther relates to one mode of an element substrate before the displayelement is completed in the manufacturing process of the semiconductordisplay device, and the element substrate is provided with a means forapplying a current or a voltage to the display element in each of aplurality of pixels. Specifically, the element substrate may be in astate in which only a pixel electrode of the display element isprovided, a state after formation of a conductive film to be a pixelelectrode and before etching of the conductive film to form the pixelelectrode, or any other states.

Embodiment 1

In this embodiment, described are results of computational scienceinvestigation on the phenomenon that a layer where the concentration ofindium is higher than that in the other regions (an In-rich layer) and atitanium oxide film (TiOx) are formed in the vicinity of the interfacebetween a metal film used as a source electrode or a drain electrode andan In—Ga—Zn—O-based oxide semiconductor film of a thin film transistorwith a channel-etched structure using the In—Ga—Zn—O-based oxidesemiconductor film as an active layer of the thin film transistor.

First, energy that is needed for formation of an oxygen-deficiency state(deficiency formation energy E_(def)) in respective case of indiumoxide, gallium oxide, and zinc oxide, which are contained in anIn—Ga—Zn—O-based oxide semiconductor, was calculated to investigatewhich metal oxide is likely to form the oxygen-deficiency state.

Note that the deficiency formation energy E_(def) is defined as Formula1 below. A represents one of the following: indium; gallium; zinc; andindium, gallium, and zinc. Note that E(O) represents half energy of anoxygen molecule, and E(A_(m)O_(n-1)) represents energy of an oxideA_(m)O_(n-1) including oxygen deficiency.

E _(def) =E(A _(m) O _(n-1))+E(O)−E(A _(m) O _(n))  (Formula 1)

Relation between the concentration of deficiency n and the deficiencyformation energy E_(def) is approximately shown as Formula 2 below. Notethat N represents the number of oxygen positions in the state wheredeficiency is not formed, k_(B) represents Boltzman constant, and Trepresents temperature.

n=N×exp(−E _(def) /k _(B) T)  (Formula 2)

For calculation, CASTEP, which is a program for a density functionaltheory, was used. A plan wave basis pseudopotential method was used as amethod for the density functional theory. GGAPBE was used for afunctional. The cut-off energy was 500 eV. The k-point sets for IGZO,In₂O₃, Ga₂O₃, and ZnO were grids of 3×3×1, 2×2×2, 2×3×2, and 4×4×1,respectively.

A crystal structure of an IGZO crystal was a structure of 84 atoms whichwas obtained by doubling a structure having a symmetry of R-3(international number: 148) in both a-axis and b-axis direction, and byarranging Ga and Zn such that the energy becomes a minimum. Crystalstructures of In₂O₃, Ga₂O₃, and ZnO are a bixbyite structure of 80atoms, a β-gallia structure of 80 atoms, and an wurtzite structure of 80atoms, respectively.

From Formula 2, it is found that as the deficiency formation energyE_(def) is increased, the concentration of oxygen deficiency n, i.e.,the amount of oxygen deficiency, is decreased. Table 1 below showsvalues of deficiency formation energy E_(def) in cases where A isindium; gallium; zinc; and indium, gallium, and zinc.

Note that the value of the deficiency formation energy E_(def) of IGZO(Model 1) is a value of the deficiency formation energy E_(def) of anoxygen atom adjacent to three indium atoms and one zinc atom in acrystal in the case where A is indium, gallium, and zinc. FIG. 4Aillustrates a structure of a portion which is formed by three indiumatoms, one zinc atom, and an oxygen atom that is adjacent to these metalatoms in an IGZO crystal.

Note also that the value of the deficiency formation energy E_(def) ofIGZO (Model 2) is a value of the deficiency formation energy E_(def) ofan oxygen atom adjacent to three indium atoms and one gallium atom in acrystal in the case where A is indium, gallium, and zinc. FIG. 4Billustrates a structure of a portion which is formed by three indiumatoms, one gallium atom, and an oxygen atom that is adjacent to thesemetal atoms in an IGZO crystal.

Note also that the value of the deficiency formation energy E_(def) ofIGZO (Model 3) is a value of the deficiency formation energy E_(def) ofan oxygen atom adjacent to two zinc atoms and two gallium atoms in acrystal in the case where A is indium, gallium, and zinc. FIG. 4Cillustrates a structure of a portion which is formed by two zinc atoms,two gallium atoms, and an oxygen atom that is adjacent to these metalatoms in an IGZO crystal.

TABLE 1 Compound E_(def) (eV) In₂O₃ 3.06 ZnO 3.75 IGZO (Model 1) 3.73IGZO (Model 2) 3.98 IGZO (Model 3) 4.08 Ga₂O₃ 4.18

As the value of deficiency formation energy E_(def) becomes high, theenergy needed for formation of an oxygen-deficiency state is increased,that is, a bond between oxygen and metal tends to be stronger.Therefore, from the values of deficiency formation energy E_(def) shownin Table 1, it is found that indium has the weakest bond with oxygen andthat oxygen is likely to be taken out in the vicinity of indium.

The oxygen-deficiency state in an In—Ga—Zn—O-based oxide semiconductoris likely to be formed because oxygen is taken out from the oxidesemiconductor by a metal used for a source electrode and a drainelectrode. Electrical conductivity of the oxide semiconductor isincreased by formation of the oxygen-deficiency state; therefore, whenoxygen is taken out in the above-described manner, electricalconductivity of an oxide semiconductor film in the vicinity of theinterface between the oxide semiconductor film and a metal film isexpected to be increased.

Next, in order to confirm whether or not oxygen is taken out from anoxide semiconductor by a metal, a quantum-mechanically stable structuremodel in the vicinity of the interface between an In—Ga—Zn—O-based oxidesemiconductor film and a metal film was investigated by calculationusing a quantum molecular dynamics (QMD) method.

A structure for calculation was manufactured in the following manner.First, a unit cell including 84 atoms of In₁₂Ga₁₂Zn₁₂O₄₈ was extractedfrom an amorphous In—Ga—Zn—O-based oxide semiconductor (a-IGZO) that wasformed by a classical molecular dynamics (CMD) method, and the structurewas optimized by quantum molecular dynamics (QMD) and a first-principlestructure optimization. By cutting the structure-optimized unit cell,a-IGZO layers were obtained. Over the a-IGZO layers, metal layers havingcrystals of respective metal atoms (W, Mo, and Ti) were stacked. Afterthat, the manufactured structures were optimized. Each of thesestructures was used as a starting object, and calculation was performedby using the quantum molecular dynamics (QMD) method at 623.0 K. Notethat the lower end of each of the a-IGZO layers and the top end of eachof the metal layers were fixed so that only interaction at the interfacecould be estimated.

Calculation conditions for the classical molecular dynamics calculationare shown below. Materials Explorer was used as a calculation program.A-IGZO was formed under the following conditions. In a calculation cellhaving a length of 1 nm on each side, 84 atoms in total (the ratio wasIn:Ga:Zn:O=1:1:1:4) were randomly arranged, and the density was set to5.9 g/cm³. The temperature was gradually lowered from 5500 K to 1 K inthe NVT ensemble. The total calculation time was 10 ns with timeintervals of 0.1 fs. Potentials between metal and oxygen, and betweenoxygen and oxygen were of a Born-Mayer-Huggins type, and a potentialbetween metal and metal was of an UFF type. Electrical charges of In,Ga, Zn, and O were +3, +3, +2, and −2, respectively.

Calculation conditions for the QMD calculation are shown below. A firstprinciple calculation software, CASTEP, was used as a calculationprogram. GGAPBE was used for a functional, and an ultrasoft type wasused for pseudopotential. The cut-off energy was 260 eV, and the k-pointset was 1×1×1. The MD calculation was performed in the NVT ensemble, andthe temperature was 623 K. The total calculation time was 2.0 ps withtime intervals of 1.0 fs.

FIGS. 5A and 5B, FIGS. 6A and 6B, and FIGS. 7A and 7B are calculationresults. In FIGS. 5A and 5B, FIGS. 6A and 6B, and FIGS. 7A and 7B, whitecircles represent any of metal atoms of W, Mo, and Ti, and black circlesrepresent oxygen atoms. FIGS. 5A and 5B illustrate structural models inthe case of using a metal layer of W. FIG. 5A illustrates the structuralmodel before calculation by the QMD method, and FIG. 5B illustrates thestructural model after the calculation by the QMD method. FIGS. 6A and6B illustrate structural models in the case of using a metal layer ofMo. FIG. 6A illustrates the structural model before calculation by theQMD method, and FIG. 6B illustrates the structural model after thecalculation by the QMD method. FIGS. 7A and 7B illustrate structuralmodels in the case of using a metal layer of Ti. FIG. 7A illustrates thestructural model before calculation by the QMD method, and FIG. 7Billustrates the structural model after the calculation by the QMDmethod.

From FIG. 6A and FIG. 7A, it is found that oxygen already transfers tothe metal layer at the time of structural optimization in the case ofusing Mo and the case of using Ti. From comparison among FIG. 5B, FIG.6B, and FIG. 7B, it is found that the largest amount of oxygen transfersin the case of using Ti. It is considered that the most suitablematerial for an electrode which causes oxygen-deficiency in a-IGZO isTi.

Oxygen that is taken out by titanium reacts with titanium, resulting intitanium oxide. Then, investigation was conducted to see whether or notthe titanium oxide film formed between the oxide semiconductor film andthe titanium film has conductivity.

Titanium dioxide can have some types of crystal structures such as arutile structure (a tetragonal system obtained at high temperature), ananatase structure (a tetragonal system obtained at low temperature), anda brookite structure (an orthorhombic system). Since the anatasestructure and the brookite structure turn into the rutile structure,which is the most stable structure, by being heated, the titanium oxidewas assumed to have the rutile structure. A crystal structure oftitanium oxide having the rutile structure is shown in FIG. 8. Therutile structure is a tetragonal system, and the space group of crystalsymmetry is P4₂/mnm.

Calculation for obtaining state density of the titanium dioxidestructure was performed by using a density functional theory using aGGAPBE functional. While symmetry was maintained, the structureincluding the cell structure was optimized and the state density wascalculated. For calculation of a density functional, a plane wavepseudopotential method in a CASTEP code was used. The cut-off energy was380 eV.

FIG. 9 shows the state density of titanium dioxide having the rutilestructure. From FIG. 9, it is found that titanium dioxide having therutile structure has a band gap, and that it has state density similarto that of an insulator or a semiconductor. Note that in the densityfunctional theory, the band gap tends to be estimated small; therefore,the actual band gap of titanium dioxide is approximately 3.0 eV, whichis larger than the band gap shown in the state density of FIG. 9.

Next, FIG. 10 shows the state density of titanium dioxide having therutile structure including oxygen deficiency. Specifically, titaniumoxide having 24 Ti atoms and 47 O atoms, which was obtained by removingone O atom from titanium oxide having 24 Ti atoms and 48 O atoms, wasused as a model for calculation. From the state density of FIG. 10, itis found that the Fermi level moves above the band gap; therefore, inthe case where oxygen deficiency is formed, titanium dioxide has n-typeconductivity.

Next, FIG. 11 shows the state density of titanium monoxide (TiO). FromFIG. 11, it is found that titanium monoxide has a state density that issimilar to that of a metal.

Therefore, from the state density of titanium dioxide in FIG. 9, thestate density of titanium dioxide including oxygen deficiency in FIG.10, and the state density of titanium monoxide in FIG. 11, it isexpected that titanium dioxide including oxygen deficiency (TiO_(2-δ))has n-type conductivity when 0<δ<1. Therefore, even in the case where atitanium oxide film contains any of titanium dioxide, titanium monoxide,and titanium dioxide including oxygen deficiency as a component, thetitanium oxide film is considered to be unlikely to inhibit current flowbetween an In—Ga—Zn—O-based oxide semiconductor film and a titaniumfilm.

FIG. 29 shows an energy band diagram between a source electrode and adrain electrode in a thin film transistor. Note that in FIG. 29, anIn—Ga—Zn—O-based non-single-crystal film (IGZO) is used as an oxidesemiconductor film, and TiOx films are included between the oxidesemiconductor film and the source electrode, and between the oxidesemiconductor film and the drain electrode of the thin film transistor.Note that the thickness of the TiOx films is more than or equal to 0.1nm and less than or equal to 10 nm. The above oxide semiconductor filmcontains a large number of metal atoms (e.g., In, Ga, and Zn) and a pairof composite layers that are in contact with the above pair of TiOxfilms. Electron affinity of the In—Ga—Zn—O-based non-single-crystal film(IGZO) in a region other than the composite layers, electron affinity ofthe TiOx films, electron affinity of Ti for the source electrode and thedrain electrode, and electron affinity of the composite layers are 4.3eV, 4.3 eV, 4.1 eV, and 4.5 eV, respectively. Note that in FIG. 29, thepositions of the bands change so that Fermi levels of the substances areequal. When a gate voltage is not applied, since the number of carriersin IGZO is small, the Fermi level is near the center of the band gap.Since the number of carriers in the TiOx films and the composite layersare large, the position of the Fermi level is close to the conductionband. Therefore, in FIG. 29, the position of a conduction band of eachsubstance differs from the above-described relative value of electronaffinity. Since there is almost no difference between the electronaffinities of the composite layers as shown in FIG. 29, it is possibleto realize a favorable connection structure between the oxidesemiconductor film and the source electrode, and between the oxidesemiconductor film and the drain electrode.

Embodiment 2

In this embodiment, a structure of a thin film transistor which includesan oxide semiconductor film in a channel formation region is describedby taking an example of a bottom-gate transistor with a channel-etchedstructure.

FIG. 1A illustrates a cross-sectional view of a thin film transistor 201and FIG. 1B illustrates a top view of the thin film transistor 201illustrated in FIG. 1A. Note that a cross-sectional view taken alongdashed line A1-A2 in FIG. 1B corresponds to FIG. 1A.

The thin film transistor 201 includes a gate electrode 203 formed over asubstrate 202 having an insulating surface, a gate insulating film 204over the gate electrode 203, an oxide semiconductor film 205 whichoverlaps with the gate electrode 203 over the gate insulating film 204and which includes composite layers 250 where the concentration of oneor a plurality of metals contained in the oxide semiconductor is higherthan that in other regions, a pair of metal oxide films 251 formed overthe oxide semiconductor film 205 and in contact with the compositelayers 250, and a source electrode 206 and a drain electrode 207 whichare in contact with the metal oxide films 251. Further, the thin filmtransistor 201 may include as its component an oxide insulating film 208formed over the oxide semiconductor film 205. The metal oxide films 251are formed by oxidation of a metal contained in the source electrode 206and the drain electrode 207.

Note that the thin film transistor 201 illustrated in FIGS. 1A to 1C hasa channel-etched structure in which part of the oxide semiconductor film205 is etched between the source electrode 206 and the drain electrode207.

An insulating film as a base film may be formed between the gateelectrode 203 and the substrate 202. The base film can be formed with asingle layer or a stacked layer using one or more of insulating filmswhich prevent diffusion of impurity elements from the substrate 202,specifically, a silicon nitride film, a silicon oxide film, a siliconnitride oxide film, and a silicon oxynitride film.

A material for the gate electrode 203 can be a single layer or a stackedlayer using one or more of a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, neodymium, or scandium, or analloy material which contains any of these metal materials as a maincomponent, or a nitride of these metals. Note that aluminum or coppercan also be used as the above metal material as long as it can withstanda temperature of heat treatment performed in a later step. Aluminum orcopper is preferably used in combination with a refractory metalmaterial in order to avoid problems with heat resistance and corrosion.As the refractory metal material, molybdenum, titanium, chromium,tantalum, tungsten, neodymium, scandium, or the like can be used.

For example, as a two-layer structure of the gate electrode 203, it ispreferable to stack a titanium nitride film and a molybdenum film. As athree-layer structure, it is preferable to stack a tungsten film or atungsten nitride film, an alloy film of aluminum and silicon or an alloyfilm of aluminum and titanium, and a titanium nitride film or a titaniumfilm.

Further, by using a light-transmitting oxide conductive film of indiumoxide, an indium oxide-tin oxide alloy, an indium oxide-zinc oxidealloy, zinc oxide, zinc aluminum oxide, zinc aluminum oxynitride, zincgallium oxide, or the like, the aperture ratio of a pixel portion can beincreased.

In this specification, oxynitride refers to a substance which containsmore oxygen than nitrogen, and nitride oxide refers to a substance whichcontains more nitrogen than oxygen.

The thickness of the gate electrode 203 is 10 nm to 400 nm, preferably100 nm to 200 nm. In this embodiment, after a conductive film with athickness of 100 nm for the gate electrode is formed by a sputteringmethod using a tungsten target, the conductive film is processed(patterned) by etching to have a desired shape, so that the gateelectrode 203 is formed.

The gate insulating film 204 can be formed with a single layer of asilicon oxide film, a silicon nitride film, a silicon oxynitride film, asilicon nitride oxide film, an aluminum oxide film, or a tantalum oxidefilm or a stacked layer thereof by a plasma CVD method, a sputteringmethod, or the like. In this embodiment, a silicon oxynitride film witha thickness of 100 nm is used as the gate insulating film 204.

After the oxide semiconductor film is formed by a sputtering methodusing an oxide semiconductor as a target, the oxide semiconductor filmis processed into a desired shape by etching or the like, so that theisland-shaped oxide semiconductor film 205 is formed. The oxidesemiconductor film can be formed by a sputtering method under a rare gas(for example, argon) atmosphere, an oxygen atmosphere, or a mixedatmosphere including a rare gas and oxygen. The thickness of theisland-shaped oxide semiconductor film 205 is more than or equal to 10nm and less than or equal to 300 nm, preferably, more than or equal to20 nm and less than or equal to 100 nm.

As the oxide semiconductor film 205, the oxide semiconductor describedabove can be used.

In this embodiment, as the oxide semiconductor film 205, anIn—Ga—Zn—O-based non-single-crystal film with a thickness of 50 nm,which is obtained by a sputtering method using an oxide semiconductortarget including indium (In), gallium (Ga), and zinc (Zn)(In₂O₃:Ga₂O₃:ZnO=1:1:1), is used.

After a conductive film for a source electrode and a drain electrode isformed over the island-shaped oxide semiconductor film 205, theconductive film is patterned by etching or the like, so that the sourceelectrode 206 and the drain electrode 207 are formed. When the sourceelectrode 206 and the drain electrode 207 are formed by the patterning,an exposed portion of the island-shaped oxide semiconductor film 205 ispartly etched in some cases. In this case, in the oxide semiconductorfilm 205, the thickness of a region between the source electrode 206 andthe drain electrode 207 becomes smaller than the thickness of regionswhich overlap with the source electrode 206 or the drain electrode 207,as illustrated in FIG. 1A.

As a material of a conductive film for the source electrode and thedrain electrode, for example, an element selected from titanium,tungsten, and molybdenum, an alloy containing one or more of the aboveelements, or the like can be used. In a semiconductor device of oneembodiment of the present invention, in the source electrode 206 and thedrain electrode 207, at least a portion which is the closest to theisland-shaped oxide semiconductor film 205 may be formed using anelement selected from titanium, tungsten, and molybdenum, an alloycontaining one or more of the above elements, or the like. Therefore, inthe case where the source electrode 206 and the drain electrode 207 eachhaving a structure in which a plurality of metal films are stacked, ametal film that is in contact with the oxide semiconductor film 205 maybe formed using titanium, tungsten, or molybdenum, and the other metalfilms can be formed using any of the following examples: an elementselected from aluminum, chromium, tantalum, titanium, manganese,magnesium, molybdenum, tungsten, zirconium, beryllium, and yttrium; analloy containing one or more of the above elements as a component; anitride containing the above element as a component; or the like. Forexample, by using a conductive film having a stacked structure of atitanium film, an aluminum alloy film containing neodymium, and atitanium film, and by using the titanium film in the portion which isthe closest to the island-shaped oxide semiconductor film 205, thesource electrode 206 and the drain electrode 207 can have a lowresistance and high heat resistance in the aluminum alloy filmcontaining neodymium.

Note that in the case where heat treatment is performed after theformation of the conductive film for the source electrode and the drainelectrode, the conductive film preferably has heat resistance enough towithstand the heat treatment. In the case of performing heat treatmentafter the formation of the conductive film, the conductive film isformed in combination with the heat-resistant conductive materialbecause aluminum alone has problems of low heat resistance, being easilycorroded, and the like. As the heat-resistant conductive material whichis combined with aluminum, the following material is preferably used: anelement selected from titanium, tantalum, tungsten, molybdenum,chromium, neodymium, and scandium; an alloy containing one or more ofthese elements as a component; a nitride containing any of theseelements as a component; or the like.

The thickness of the conductive film for the source electrode and thedrain electrode is 10 nm to 400 nm, preferably 100 nm to 200 nm. In thisembodiment, after a conductive film for a source electrode and a drainelectrode is formed by a sputtering method using a titanium target, theconductive film is processed (patterned) by etching to have a desiredshape, so that the source electrode 206 and the drain electrode 207 areformed.

By forming the source electrode 206 and the drain electrode 207 havingthe above structure, oxygen in the region of the oxide semiconductorfilm 205 which is the closest to the source electrode 206 and the drainelectrode 207 is taken out, so that the composite layers 250 where theconcentration of a metal contained in the oxide semiconductor film 205is higher than that in other regions (metal-rich layers) are formed inthe oxide semiconductor film 205. The oxygen that is taken out reactswith the metal in the source electrode 206 and the drain electrode 207,so that the metal oxide films 251 are formed between the metal-richcomposite layer 250 and the source electrode 206, and between themetal-rich composite layer 250 and the drain electrode 207. Thethickness of the metal-rich composite layers 250 is more than or equalto 2 nm and less than or equal to 10 nm, and the thickness of the metaloxide films 251 is more than or equal to 2 nm and less than or equal to10 nm.

For example, in the case where an In—Ga—Zn—O-based oxide semiconductoris used for the oxide semiconductor film 205, the composite layers 250where the concentration of indium is higher than that in other regions(In-rich layers) exist in regions of the oxide semiconductor film 205which are the closest to the source electrode 206 and the drainelectrode 207, so that resistance of the In-rich composite layers 250 inthe oxide semiconductor film 205 becomes lower. In the case wheretitanium is used for the source electrode 206 and the drain electrode207, the metal oxide films 251 formed between the source electrode 206and the oxide semiconductor film 205 and between the drain electrode 207and the oxide semiconductor film 205 contain titanium oxide (TiOx) andhave n-type conductivity. Therefore, with the above structure, contactresistance between the source electrode 206 and the oxide semiconductorfilm 205 and between the drain electrode 207 and the oxide semiconductorfilm 205 is reduced, and the amount of on-current and field effectmobility of a TFT can be increased.

The oxide insulating film 208 is formed to be in contact with theisland-shaped oxide semiconductor film 205, the source electrode 206,and the drain electrode 207 by a sputtering method. The oxide insulatingfilm 208 in contact with the island-shaped oxide semiconductor film 205is preferably formed using an inorganic insulating film which containsas few impurities, e.g., moisture, hydrogen, and a hydroxy group, aspossible and blocks entry of these impurities from the outside, such asa silicon oxide film, a silicon nitride oxide film, an aluminum oxidefilm, or an aluminum oxynitride film. In this embodiment, a siliconoxide film with a thickness of 300 nm is preferably formed as the oxideinsulating film 208.

When the oxide insulating film 208 is formed in contact with the oxidesemiconductor film 205 by a sputtering method, a PCVD method, or thelike, oxygen is supplied to at least a region of the oxide semiconductorfilm 205 which is in contact with the oxide insulating film 208, andresistance becomes higher because the carrier concentration becomes low,preferably to a value of less than 1×10¹⁸/cm³; as a result, ahigh-resistance oxide semiconductor region is formed. By forming theoxide insulating film 208, the oxide semiconductor film 205 has ahigh-resistance oxide semiconductor region in vicinity of an interfacebetween the oxide semiconductor film 205 and the oxide insulating film208.

Note that as illustrated in FIG. 1C, the thin film transistor 201 mayfurther include a conductive film 209 over the oxide insulating film208. A material or stacked layer structure similar to that for the gateelectrode 203 can be used for the conductive film 209. The thickness ofthe conductive film 209 is 10 nm to 400 nm, preferably 100 nm to 200 nm.A resist mask is formed by a photolithography method and a conductivefilm is processed (patterned) to have a desired shape. The conductivefilm 209 is formed so as to overlap with a channel formation region inthe oxide semiconductor film 205. The conductive film 209 may be in afloating state, that is, electrically insulated, or may be in a state inwhich a potential is given. In the latter case, a potential having thesame level as the gate electrode 203 or a fixed potential such as aground potential may be given to the conductive film 209. By controllingthe level of a potential given to the conductive film 209, the thresholdvoltage of the thin film transistor 201 can be controlled.

Further, in the case of forming the conductive film 209, an insulatingfilm 210 is formed so as to cover the conductive film 209. Theinsulating film 210 is formed using an inorganic insulating film whichcontains as few impurities, e.g., moisture, hydrogen, and a hydroxygroup, as possible and blocks entry of these impurities from theoutside, such as a silicon oxide film, a silicon nitride oxide film, analuminum oxide film, or an aluminum oxynitride film.

A thin film transistor using an oxide semiconductor has high mobilitycompared to a thin film transistor using amorphous silicon, and uniformelement characteristics similar to those of a thin film transistor usingamorphous silicon. Accordingly, an oxide semiconductor can be used fornot only a pixel portion but also a semiconductor element which forms adriver circuit with higher driving frequency than the pixel portion. Asystem-on-panel can be realized without a process of crystallization orthe like.

This embodiment can be implemented in combination with the aboveembodiment.

Embodiment 3

In this embodiment, a structure of a bottom-contact thin film transistorwhich is different from that of the thin film transistor 201 illustratedin Embodiment 2 is described. For the same portions as those inEmbodiment 2 or portions having functions similar to those in Embodiment2, Embodiment 2 can be referred to, and repetitive description thereofis omitted.

FIG. 12A illustrates a cross-sectional view of a thin film transistor211, and FIG. 12B illustrates a top view of the thin film transistor 211illustrated in FIG. 12A. Note that a cross-sectional view taken alongdashed line B1-B2 in FIG. 12B corresponds to FIG. 12A.

The thin film transistor 211 includes a gate electrode 213 formed over asubstrate 212 having an insulating surface, a gate insulating film 214over the gate electrode 213, a source electrode 216 or a drain electrode217 over the gate insulating film 214, metal oxide films 261 in contactwith the source electrode 216 or the drain electrode 217, and an oxidesemiconductor film 215 which overlaps with the gate electrode 213 andwhich includes composite layers 260 where the concentration of one or aplurality of metals contained in the oxide semiconductor is higher thanthat in other regions. The composite layers 260 are in contact with themetal oxide films 261. Further, the thin film transistor 211 may includeas its component an oxide insulating film 218 formed over the oxidesemiconductor film 215. The metal oxide films 261 are formed byoxidation of a metal contained in the source electrode 216 and the drainelectrode 217.

An insulating film as a base film may be provided between the gateelectrode 213 and the substrate 212. The base film can be formed using amaterial and a stacked layer structure similar to those in Embodiment 2.In addition, the gate electrode 213 can be formed using the material andstacked layer structure similar to those in Embodiment 2.

The thickness of the gate electrode 213 is 10 nm to 400 nm, preferably100 nm to 200 nm. In this embodiment, after a conductive film with athickness of 100 nm for the gate electrode is formed by a sputteringmethod using a tungsten target, the conductive film is processed(patterned) by etching to have a desired shape, so that the gateelectrode 213 is formed.

The gate insulating film 214 can be formed using the material andstacked layer structure similar to those in Embodiment 2, and themanufacturing method shown in Embodiment 2. In this embodiment, asilicon oxynitride film with a thickness of 100 nm is used as the gateinsulating film 204.

After a conductive film for a source electrode and a drain electrode isformed over the gate insulating film 214, the conductive film ispatterned by etching or the like, so that the source electrode 216 andthe drain electrode 217 are formed.

As a material of a conductive film for the source electrode and thedrain electrode, for example, an element selected from titanium,tungsten, and molybdenum, an alloy containing one or more of the aboveelements, or the like can be used. In a semiconductor device of oneembodiment of the present invention, in the source electrode 216 and thedrain electrode 217, at least a portion which is the closest to theisland-shaped oxide semiconductor film 215 to be formed later may beformed using an element selected from titanium, tungsten, andmolybdenum, an alloy containing one or more of the above elements, orthe like. Therefore, in the case where the source electrode 216 and thedrain electrode 217 each having a structure in which a plurality ofmetal films are stacked, a metal film that is in contact with the oxidesemiconductor film 215 may be formed using titanium, tungsten, ormolybdenum, and the other metal films can be formed using any of thefollowing examples: an element selected from aluminum, chromium,tantalum, titanium, manganese, magnesium, molybdenum, tungsten,zirconium, beryllium, and yttrium; an alloy containing one or more ofthe above elements as a component; a nitride containing the aboveelement as a component; or the like. For example, by using a conductivefilm having a stacked structure of a titanium film, an aluminum alloyfilm containing neodymium, and a titanium film, and by using thetitanium film in the portion which is the closest to the island-shapedoxide semiconductor film 215, the source electrode 216 and the drainelectrode 217 can have a low resistance and high heat resistance in thealuminum alloy film containing neodymium.

Note that in the case where heat treatment is performed after theformation of the conductive film for the source electrode and the drainelectrode, the conductive film preferably has heat resistance enough towithstand the heat treatment. In the case of performing heat treatmentafter the formation of the conductive film, the conductive film isformed in combination with the heat-resistant conductive materialbecause aluminum alone has problems of low heat resistance, being easilycorroded, and the like. As the heat-resistant conductive material whichis combined with aluminum, the following material is preferably used: anelement selected from titanium, tantalum, tungsten, molybdenum,chromium, neodymium, and scandium; an alloy containing one or more ofthese elements as a component; a nitride containing any of theseelements as a component; or the like.

The source electrode 216 and the drain electrode 217 of a bottom-contactthin film transistor are preferably thinner than those of thebottom-gate transistor illustrated in Embodiment 2 in order to preventbreakage of the oxide semiconductor film 215 formed later. Specifically,the thicknesses of the source electrode 216 and the drain electrode 217are 10 nm to 200 nm, preferably 50 nm to 75 nm. In this embodiment,after a conductive film for a source electrode and a drain electrode isformed by a sputtering method using a titanium target, the conductivefilm is processed (patterned) to have a desired shape by etching, sothat the source electrode 216 and the drain electrode 217 are formed.

The island-shaped oxide semiconductor film 215 can be formed using amaterial similar to that in Embodiment 2 and the manufacturing methodshown in Embodiment 2, so as to be in contact with the gate insulatingfilm 214 in a position overlapping with the gate electrode 213 over thesource electrode 216 and the drain electrode 217.

In this embodiment, as the oxide semiconductor film 215, anIn—Ga—Zn—O-based non-single-crystal film with a thickness of 50 nm,which is obtained by a sputtering method using an oxide semiconductortarget including indium (In), gallium (Ga), and zinc (Zn)(In₂O₃:Ga₂O₃:ZnO=1:1:1), is used.

By forming the oxide semiconductor film 215 having the above structureover the source electrode 216 and the drain electrode 217, oxygen in theregion of the oxide semiconductor film 215 which is the closest to thesource electrode 216 and the drain electrode 217 is taken out, so thatthe composite layers 260 where the concentration of a metal contained inthe oxide semiconductor film 215 is higher than that in other regions(metal-rich layers) are formed in the oxide semiconductor film 215. Theoxygen that is taken out reacts with the metal in the source electrode216 and the drain electrode 217, so that the metal oxide films 261 areformed between the metal-rich composite layer 260 and the sourceelectrode 216, and between the metal-rich composite layer 260 and thedrain electrode 217. The thickness of the metal-rich composite layers260 is more than or equal to 2 nm and less than or equal to 10 nm, andthe thickness of the metal oxide films 261 is more than or equal to 2 nmand less than or equal to 10 nm.

For example, in the case where an In—Ga—Zn—O-based oxide semiconductoris used for the oxide semiconductor film 215, the composite layers 260where the concentration of indium is higher than that in other regions(In-rich layers) exist in regions of the oxide semiconductor film 215which are the closest to the source electrode 216 and the drainelectrode 217, so that resistance of the In-rich composite layers 260 inthe oxide semiconductor film 215 becomes lower. In the case wheretitanium is used for the source electrode 216 and the drain electrode217, the metal oxide films 261 formed between the source electrode 216and the oxide semiconductor film 215, and between the drain electrode217 and the oxide semiconductor film 215 contain titanium oxide (TiOx)and have n-type conductivity. Therefore, with the above structure,contact resistance between the source electrode 216 and the oxidesemiconductor film 215, and between the drain electrode 217 and theoxide semiconductor film 215 is reduced, and the amount of on-currentand field effect mobility of a TFT can be increased.

The oxide insulating film 218 is formed to be in contact with theisland-shaped oxide semiconductor film 215 by a sputtering method. Theoxide insulating film 218 can be formed using the material and stackedlayer structure similar to those in Embodiment 2, and the manufacturingmethod shown in Embodiment 2. In this embodiment, a silicon oxide filmwith a thickness of 300 nm is formed as the oxide insulating film 218.

Note that as illustrated in FIG. 12C, the thin film transistor 211 mayfurther include a conductive film 219 over the oxide insulating film218. A material or stacked layer structure similar to that for the gateelectrode 213 can be used for the conductive film 219. The thickness ofthe conductive film 219 is 10 nm to 400 nm, preferably 100 nm to 200 nm.A resist mask is formed by a photolithography method and a conductivefilm is processed (patterned) to have a desired shape. The conductivefilm 219 is formed so as to overlap with a channel formation region inthe oxide semiconductor film 215. The conductive film 219 may be in afloating state, that is, electrically insulated, or may be in a state inwhich a potential is given. In the latter case, a potential having thesame level as the gate electrode 213 or a fixed potential such as aground potential may be given to the conductive film 219. By controllingthe level of a potential given to the conductive film 219, the thresholdvoltage of the thin film transistor 211 can be controlled.

Further, in the case of forming the conductive film 219, an insulatingfilm 220 is formed so as to cover the conductive film 219. Theinsulating film 220 is formed using an inorganic insulating film whichcontains as few impurities, e.g., moisture, hydrogen, and a hydroxygroup, as possible and blocks entry of these impurities from theoutside, such as a silicon oxide film, a silicon nitride oxide film, analuminum oxide film, or an aluminum oxynitride film.

A thin film transistor using an oxide semiconductor has high mobilitycompared to a thin film transistor using amorphous silicon, and uniformelement characteristics similar to those of a thin film transistor usingamorphous silicon. Accordingly, an oxide semiconductor can be used fornot only a pixel portion but also a semiconductor element which forms adriver circuit with higher driving frequency than the pixel portion. Asystem-on-panel can be realized without a process of crystallization orthe like.

This embodiment can be implemented in combination with any of the aboveembodiments.

Embodiment 4

In this embodiment, a structure of a bottom-gate thin film transistorwith a channel-protective structure which is different from that of thethin film transistor 201 illustrated in Embodiment 2 or the thin filmtransistor 211 illustrated in Embodiment 3 is described. For the sameportions as those in Embodiment 2 or portions having functions similarto those in Embodiment 2, Embodiment 2 can be referred to, andrepetitive description thereof is omitted.

FIG. 13A illustrates a cross-sectional view of a thin film transistor221, and FIG. 13B illustrates a top view of the thin film transistor 221illustrated in FIG. 13A. Note that a cross-sectional view taken alongdashed line C1-C2 in FIG. 13B corresponds to FIG. 13A.

The thin film transistor 221 includes a gate electrode 223 formed over asubstrate 222 having an insulating surface, a gate insulating film 224over the gate electrode 223, an oxide semiconductor film 225 whichoverlaps with the gate electrode 223 over the gate insulating film 224and which includes composite layers 270 where the concentration of oneor a plurality of metals contained in the oxide semiconductor is higherthan that in other regions, a pair of metal oxide films 271 formed overthe oxide semiconductor film 225 and in contact with the compositelayers 270, a source electrode 226 and a drain electrode 227 which arein contact with the metal oxide films 271, and a channel protective film231 formed over the island-shaped oxide semiconductor film 225 in aposition overlapping with the gate electrode 223. Further, the thin filmtransistor 221 may include as its component an oxide insulating film 228formed over the oxide semiconductor film 225. The metal oxide films 271are formed by oxidation of a metal contained in the source electrode 226and the drain electrode 227.

An insulating film as a base film may be provided between the gateelectrode 223 and the substrate 222. The base film can be formed using amaterial and a stacked layer structure similar to those in Embodiment 2.In addition, the gate electrode 223 can be formed using the material andstacked layer structure similar to those in Embodiment 2.

The thickness of the gate electrode 223 is 10 nm to 400 nm, preferably100 nm to 200 nm. In this embodiment, after a conductive film with athickness of 100 nm for the gate electrode is formed by a sputteringmethod using a tungsten target, the conductive film is processed(patterned) by etching to have a desired shape, so that the gateelectrode 223 is formed.

The gate insulating film 224 can be formed using the material andstacked layer structure similar to those in Embodiment 2, and themanufacturing method shown in Embodiment 2. In this embodiment, asilicon oxynitride film with a thickness of 100 nm is used as the gateinsulating film 224.

The island-shaped oxide semiconductor film 225 can be formed by usingthe same material as in Embodiment 2 and the method described inEmbodiment 2, over the gate insulating film 224 in a position whichoverlaps with the gate electrode 223.

In this embodiment, as the oxide semiconductor film 225, anIn—Ga—Zn—O-based non-single-crystal film with a thickness of 50 nm,which is obtained by a sputtering method using an oxide semiconductortarget including indium (In), gallium (Ga), and zinc (Zn)(In₂O₃:Ga₂O₃:ZnO=1:1:1), is used.

The channel protective film 231 is formed over the island-shaped oxidesemiconductor film 225 in a position of the island-shaped oxidesemiconductor film 225 which overlaps with a portion to be a channelformation region, i.e., a position which overlaps with the gateelectrode 223. The channel protective film 231 can prevent the portionof the oxide semiconductor film 225, which serves as a channel formationregion later, from being damaged in a later step (for example, reductionin thickness due to plasma or an etchant in etching). Therefore,reliability of the thin film transistor can be improved.

The channel protective film 231 can be formed using an inorganicmaterial that contains oxygen (e.g., silicon oxide, silicon nitrideoxide, silicon oxynitride, aluminum oxide, or aluminum oxynitride). Thechannel protective film 231 can be formed by a vapor deposition methodsuch as a plasma CVD method or a thermal CVD method, or a sputteringmethod. After the formation of the channel protective film 231, theshape thereof is processed by etching. Here, the channel protective film231 is formed in such a manner that a silicon oxide film is formed by asputtering method and processed by etching using a mask formed byphotolithography.

When the channel protective film 231, which is an oxide insulating film,is formed to be in contact with the island-shaped oxide semiconductorfilm 225 by a sputtering method, a PCVD method, or the like, oxygen issupplied from the channel protective film 231. Carrier concentration atleast in a region of the island-shaped oxide semiconductor film 225 incontact with the channel protective film 231 is preferably lowered toless than 1×10¹⁸/cm³, more preferably equal to or less than 1×10¹⁴/cm³,and resistance becomes higher, resulting in a high-resistance oxidesemiconductor region. By formation of the channel protective film 231,the oxide semiconductor film 225 can have the high-resistance oxidesemiconductor region in the vicinity of the interface between the oxidesemiconductor film 225 and the channel protective film 231.

After a conductive film for a source electrode and a drain electrode isformed over the island-shaped oxide semiconductor film 225 and thechannel protective film 231, the conductive film is patterned by etchingor the like, so that the source electrode 226 and the drain electrode227 are formed.

As a material of a conductive film for the source electrode and thedrain electrode, for example, an element selected from titanium,tungsten, and molybdenum, an alloy containing one or more of the aboveelements, or the like can be used. In a semiconductor device of oneembodiment of the present invention, in the source electrode 226 and thedrain electrode 227, at least a portion which is the closest to theisland-shaped oxide semiconductor film 225 may be formed using anelement selected from titanium, tungsten, and molybdenum, an alloycontaining one or more of the above elements, or the like. Therefore, inthe case where the source electrode 226 and the drain electrode 227 eachhaving a structure in which a plurality of metal films are stacked, ametal film that is in contact with the oxide semiconductor film 225 maybe formed using titanium, tungsten, or molybdenum, and the other metalfilms can be formed using any of the following examples: an elementselected from aluminum, chromium, tantalum, titanium, manganese,magnesium, molybdenum, tungsten, zirconium, beryllium, and yttrium; analloy containing one or more of the above elements as a component; anitride containing the above element as a component; or the like. Forexample, by using a conductive film having a stacked structure of atitanium film, an aluminum alloy film containing neodymium, and atitanium film, and by using the titanium film in the portion which isthe closest to the island-shaped oxide semiconductor film 225, thesource electrode 226 and the drain electrode 227 can have a lowresistance and high heat resistance in the aluminum alloy filmcontaining neodymium.

Note that in the case where heat treatment is performed after theformation of the conductive film for the source electrode and the drainelectrode, the conductive film preferably has heat resistance enough towithstand the heat treatment. In the case of performing heat treatmentafter the formation of the conductive film, the conductive film isformed in combination with the heat-resistant conductive materialbecause aluminum alone has problems of low heat resistance, being easilycorroded, and the like. As the heat-resistant conductive material whichis combined with aluminum, the following material is preferably used: anelement selected from titanium, tantalum, tungsten, molybdenum,chromium, neodymium, and scandium; an alloy containing one or more ofthese elements as a component; a nitride containing any of theseelements as a component; or the like.

The thickness of the conductive film for the source electrode and thedrain electrode is 10 nm to 400 nm, preferably 100 nm to 200 nm. In thisembodiment, after the conductive film for the source electrode and thedrain electrode is formed by a sputtering method using a titaniumtarget, the conductive film is processed (patterned) by etching to havea desired shape, so that the source electrode 226 and the drainelectrode 227 are formed.

By forming the source electrode 226 and the drain electrode 227 havingthe above structure, oxygen in the region of the oxide semiconductorfilm 225 which is the closest to the source electrode 226 and the drainelectrode 227 is taken out, so that the composite layers 270 where theconcentration of a metal contained in the oxide semiconductor film 225is higher than that in other regions (metal-rich layers) are formed inthe oxide semiconductor film 225. The oxygen that is taken out reactswith the metal in the source electrode 226 and the drain electrode 227,so that the metal oxide films 271 are formed between the metal-richcomposite layer 270 and the source electrode 226, and between themetal-rich composite layer 270 and the drain electrode 227. Thethickness of the metal-rich composite layers 270 is more than or equalto 2 nm and less than or equal to 10 nm, and the thickness of the metaloxide films 271 is more than or equal to 2 nm and less than or equal to10 nm.

For example, in the case where an In—Ga—Zn—O-based oxide semiconductoris used for the oxide semiconductor film 225, the composite layers 270where the concentration of indium is higher than that in other regions(In-rich layers) exist in regions of the oxide semiconductor film 225which are the closest to the source electrode 226 and the drainelectrode 227, so that resistance of the In-rich composite layers 270 inthe oxide semiconductor film 225 becomes lower. In the case wheretitanium is used for the source electrode 226 and the drain electrode227, the metal oxide films 271 formed between the source electrode 226and the oxide semiconductor film 225, and between the drain electrode227 and the oxide semiconductor film 225 contain titanium oxide (TiOx)and have n-type conductivity. Therefore, with the above structure,contact resistance between the source electrode 226 and the oxidesemiconductor film 225, and between the drain electrode 227 and theoxide semiconductor film 225 is reduced, and the amount of on-currentand field effect mobility of a TFT can be increased.

The oxide insulating film 228 is formed to be in contact with the sourceelectrode 226 and the drain electrode 227 by a sputtering method. Theoxide insulating film 228 can be formed using the material and stackedlayer structure similar to those in Embodiment 2, and the manufacturingmethod shown in Embodiment 2. Note that when the channel protective film231 is formed, the oxide insulating film 228 is not necessarily formed.

Note that as illustrated in FIG. 13C, the thin film transistor 221 mayfurther include a conductive film 229 over the oxide insulating film228. A material or stacked layer structure similar to that for the gateelectrode 223 can be used for the conductive film 229. The thickness ofthe conductive film 229 is 10 nm to 400 nm, preferably 100 nm to 200 nm.A resist mask is formed by a photolithography method and a conductivefilm is processed (patterned) to have a desired shape. The conductivefilm 229 is formed so as to overlap with a channel formation region inthe oxide semiconductor film 225. The conductive film 229 may be in afloating state, that is, electrically insulated, or may be in a state inwhich a potential is given. In the latter case, a potential having thesame level as the gate electrode 223 or a fixed potential such as aground potential may be given to the conductive film 229. By controllingthe level of a potential given to the conductive film 229, the thresholdvoltage of the thin film transistor 221 can be controlled.

Further, in the case of forming the conductive film 229, an insulatingfilm 230 is formed so as to cover the conductive film 229. Theinsulating film 230 is formed using an inorganic insulating film whichcontains as few impurities, e.g., moisture, hydrogen, and a hydroxygroup, as possible and blocks entry of these impurities from theoutside, such as a silicon oxide film, a silicon nitride oxide film, analuminum oxide film, or an aluminum oxynitride film.

A thin film transistor using an oxide semiconductor has high mobilitycompared to a thin film transistor using amorphous silicon, and uniformelement characteristics similar to those of a thin film transistor usingamorphous silicon. Accordingly, an oxide semiconductor can be used fornot only a pixel portion but also a semiconductor element which forms adriver circuit with higher driving frequency than the pixel portion. Asystem-on-panel can be realized without a process of crystallization orthe like.

This embodiment can be implemented in combination with any of the aboveembodiments.

Embodiment 5

In this embodiment, a structure of a semiconductor display devicereferred to as an electronic paper or a digital paper, which is asemiconductor display device of the present invention, is described.

A display element which can control a grayscale by voltage applicationand has a memory property is used for the electronic paper.Specifically, in the display element used for the electric paper, adisplay element such as a non-aqueous electrophoretic display element; adisplay element which uses a PDLC (polymer dispersed liquid crystal)method, in which liquid crystal droplets are dispersed in a high polymermaterial which is between two electrodes; a display element whichincludes chiral nematic liquid crystal or cholesteric liquid crystalbetween two electrodes; a display element which includes charged fineparticles between two electrodes and employs a particle-moving method inwhich the charged fine particles are moved through fine particles byusing an electric field; or the like can be used. Further, a non-aqueouselectrophoretic display element may be a display element in which adispersion liquid in which charged fine particles are dispersed isinterposed between two electrodes; a display element in which adispersion liquid in which charged fine particles are dispersed isincluded over two electrodes between which an insulating film isinterposed; a display element in which twisting balls having hemisphereswhich are different colors which charge differently are dispersed in asolvent between two electrodes; a display element which includesmicrocapsules in which a plurality of charged fine particles aredispersed in a solution, between two electrodes; or the like.

FIG. 14A illustrates a top view of a pixel portion 700, a signal linedriver circuit 701, and a scan line driver circuit 702 of an electronicpaper.

The pixel portion 700 includes a plurality of pixels 703. Further, aplurality of signal lines 707 are led into the pixel portion 700 fromthe signal line driver circuit 701. A plurality of scan lines 708 areled into the pixel portion 700 from the scan line driver circuit 702.

Each of the pixels 703 includes a transistor 704, a display element 705,and a storage capacitor 706. A gate electrode of the transistor 704 isconnected to one of the scan lines 708. Further, one of a sourceelectrode and a drain electrode of the transistor 704 is connected toone of the signal lines 707 and the other of the source electrode andthe drain electrode of the transistor 704 is connected to a pixelelectrode of the display element 705.

Note that in FIG. 14A, the storage capacitor 706 is connected inparallel to the display element 705 so that a voltage applied betweenthe pixel electrode and a counter electrode of the display element 705is stored; however, in the case where the memory property of the displayelement 705 is so high that display can be maintained, the storagecapacitor 706 is not necessarily provided.

Note that although a structure of an active matrix pixel portion inwhich one transistor which serves as a switching element is provided ineach pixel is described in FIG. 14A, the electric paper which is oneembodiment of the invention is not limited to this structure. Aplurality of transistors may be provided in each pixel. Further, inaddition to transistors, elements such as capacitors, resistors, coils,or the like may also be connected.

An electronic paper of an electrophoretic system including microcapsulesis given as one example. FIG. 14B illustrates a cross-sectional view ofthe display element 705 provided for each of the pixels 703.

The display element 705 includes a pixel electrode 710, a counterelectrode 711, and microcapsules 712 to which a voltage is applied bythe pixel electrode 710 and the counter electrode 711. Either the sourceelectrode or the drain electrode 713 of a transistor 704 is connected tothe pixel electrode 710.

In the microcapsules 712, positively charged white pigment such astitanium oxide and negatively charged black pigment such as carbon blackare sealed together with a dispersion medium such as oil. A voltage isapplied between the pixel electrode and the counter electrode inaccordance with the voltage of a video signal applied to the pixelelectrode 710, and black pigment and white pigment are drawn to apositive electrode side and a negative electrode side, respectively.Thus, the grayscale can be displayed.

Further, in FIG. 14B, the microcapsules 712 are fixed bylight-transmitting resin 714 between the pixel electrode 710 and thecounter electrode 711. However, the present invention is not limited tothis structure. A space formed by the microcapsules 712, the pixelelectrode 710, and the counter electrode 711 may be filled with gas suchas inert gas or air. Note that in this case, the microcapsules 712 ispreferably fixed to both or either the pixel electrode 710 and/or thecounter electrode 711 by an adhesive or the like.

In addition, the number of the microcapsules 712 included in the displayelement 705 is not necessarily plural as in FIG. 14B. One displayelement 705 may include a plurality of microcapsules 712 or a pluralityof display elements 705 may include one microcapsule 712. For example,two display elements 705 share one microcapsule 712, and a positivevoltage and a negative voltage are applied to the pixel electrode 710included in one of the display elements 705 and the pixel electrode 710included in the other of the display elements 705, respectively. In thiscase, in the microcapsule 712 in a region overlapping with the pixelelectrode 710 to which a positive voltage is applied, black pigment isdrawn to the pixel electrode 710 side and white pigment is drawn to thecounter electrode 711 side. In contrast, in the microcapsule 712 in aregion overlapping with the pixel electrode 710 to which a negativevoltage is applied, white pigment is drawn to the pixel electrode 710side and black pigment is drawn to the counter electrode 711 side.

Next, a specific driving method of an electronic paper is described bytaking an example of the above electronic paper of the electrophoreticsystem.

Operation of the electronic paper in an initialization period, a writingperiod, and a holding period can be separately described.

First, in the initialization period before a display image is switched,the grayscale levels of each of the pixels in a pixel portion aretemporarily set to be equal in order to initialize display elements.Initialization of the gray scale level prevents an afterimage.Specifically, in an electrophoretic system, displayed grayscale level isadjusted by the microcapsule 712 included in the display element 705such that the display of each pixel is white or black.

In this embodiment, an operation of initialization in the case whereafter an initialization video signal for displaying black is inputted toa pixel, an initialization video signal for displaying white is inputtedto a pixel is described. For example, when the electronic paper of anelectrophoretic system in which display of an image is performed towardthe counter electrode 711 side, a voltage is applied to the displayelement 705 such that black pigment in the microcapsule 712 moves to thecounter electrode 711 side and white pigment in the microcapsule 712moves to the pixel electrode 710 side. Next, a voltage is applied to thedisplay element 705 such that white pigment in the microcapsule 712moves to the counter electrode 711 side and black pigment in themicrocapsule 712 moves to the pixel electrode 710 side.

Further, depending on the grayscale level displayed before theinitialization period, only one-time input of an initialization videosignal to the pixel could possibly stop the move of white pigment andblack pigment in the microcapsule 712 and cause a difference betweendisplayed grayscale levels of pixels even after the initializationperiod ends. Therefore, it is preferable that a negative voltage −Vpwith respect to a common voltage Vcom be applied to the pixel electrode710 a plurality of times so that black is displayed and a positivevoltage Vp with respect to the common voltage Vcom be applied to thepixel electrode 710 a plurality of times so that white is displayed.

Note that when grayscale levels displayed before the initializationperiod differ depending on display elements of each of the pixels, theminimum number of times for inputting an initialization video signalalso varies. Accordingly, the number of times for inputting aninitialization video signal may be changed between pixels in accordancewith a grayscale level displayed before the initialization period. Inthis case, the common voltage Vcom is preferably inputted to a pixel towhich the initialization video signal is not necessarily inputted.

Note that in order for the voltage Vp or the voltage −Vp which is aninitialization video signal to be applied to the pixel electrode 710 aplurality of times, the following operation sequence is performed aplurality of times: the initialization video signal is inputted to apixel of a line including the scan line in a period during which a pulseof a selection signal is supplied to each scan line. The voltage Vp orthe voltage −Vp of an initialization video signal is applied to thepixel electrode 710 a plurality of times, whereby movement of whitepigment and black pigment in the microcapsule 712 converges in order toprevent generation of a difference of grayscale levels between pixels.Thus, initialization of a pixel in the pixel portion can be performed.

Note that in each pixel in the initialization period, the case whereblack is displayed after white as well as the case where white isdisplayed after black is acceptable. Alternatively, in each pixel in theinitialization period, the case where black is displayed after white isdisplayed; and further, after that white is displayed is alsoacceptable.

Further, as for all of the pixels in the pixel portion, timing ofstarting the initialization period is not necessarily the same. Forexample, timing of starting the initialization period may be differentfor every pixel, or every pixel belonging to the same line, or the like.

Next in the writing period, a video signal having image data is inputtedto the pixel.

In the case where an image is displayed on the entire pixel portion, inone frame period, a selection signal in which a pulse of voltage isshifted is sequentially inputted to all of the scan lines. Then, in oneline period in which a pulse appears in a selection signal, a videosignal having image data is inputted to all of the signal line.

White pigment and black pigment in the microcapsule 712 are moved to thepixel electrode 710 side and the counter electrode 711 in accordancewith the voltage of the video signal applied to the pixel electrode 710,so that the display element 705 displays a grayscale.

Note that also in the writing period, the voltage of a video signal ispreferably applied to the pixel electrode 710 a plurality of times as inthe initialization period. Accordingly, the following operation sequenceis performed a plurality of times: the video signal is inputted to apixel of a line including the scan line in a period during which a pulseof a selection signal is supplied to each scan line.

Next, in the holding period, after the common voltage Vcom is inputtedto all of the pixels through signal lines, a selection signal is notinputted to a scan line, or a video signal is not inputted to a signalline. Accordingly, the positions of white pigment and black pigment inthe microcapsule 712 included in the display element 705 is maintainedunless a positive or negative voltage is applied between the pixelelectrode 710 and the counter electrode 711, so that the grayscale leveldisplayed on the display element 705 is held. Therefore, an imagewritten in the writing period is maintained even in the holding period.

Note that a voltage needed for changing gray scales of the displayelement used for an electric paper tends to be higher than that of aliquid crystal element used for a liquid crystal display device or thatof a light-emitting element such as an organic light-emitting elementused for a light-emitting device. Therefore, the potential differencebetween the source electrode and the drain electrode of the transistor704 of a pixel serving for a switching element in a writing period islarge; as a result, off-current is increased, and disturbance of displayis likely to occur due to fluctuation of potentials of the pixelelectrode 710. In order to prevent fluctuation of potentials of thepixel electrode 710 caused by the off-current of the transistor 704, itis effective to increase capacitance of the storage capacitor 706. Inaddition, noise of display by the display element 705 can occur in somecases by not only a voltage between the pixel electrode 710 and thecounter electrode 711, but also a voltage generated between the signalline 707 and the counter electrode 711 being applied to microcapsules712. In order to prevent the noise, it is effective to secure a largearea of the pixel electrode 710 and prevent the voltage generatedbetween the signal line 707 and the counter electrode 711 from beingapplied to the microcapsules 712. However, as described above, whencapacitance of the storage capacitor 706 is increased in order toprevent fluctuations of potentials of the pixel electrode 710, or whenthe area of the pixel electrode 710 is increased in order to prevent thenoise of display, the value of current to be supplied to a pixel in awiring period becomes high, resulting in a longer time for input of avideo signal. In an electric paper of one embodiment of the presentinvention, since the transistor 704 used for a pixel as a switchingelement has high field effect mobility, a high on-current can beobtained. As a result, even when the capacitance of the storagecapacitor 706 is increased, or even when the area of the pixel electrode710 is increased, a video signal can be rapidly input to a pixel.Therefore, the length of the writing time can be suppressed, anddisplayed images can be smoothly switched.

This embodiment can be implemented in combination with any of the aboveembodiments.

Embodiment 6

FIG. 15A is an example of a block diagram of an active matrixsemiconductor display device. Over a substrate 5300 in the displaydevice, a pixel portion 5301, a first scan line driver circuit 5302, asecond scan line driver circuit 5303, and a signal line driver circuit5304 are provided. In the pixel portion 5301, a plurality of signallines extended from the signal line driver circuit 5304 is arranged anda plurality of scan lines extended from the first scan line drivercircuit 5302 and the second scan line driver circuit 5303 is arranged.Note that pixels which include display elements are provided in a matrixin respective regions where the scan lines and the signal linesintersect with each other. Further, the substrate 5300 in the displaydevice is connected to a timing control circuit 5305 (also referred toas a controller or a controller IC) through a connection portion such asa flexible printed circuit (FPC).

In FIG. 15A, the first scan line driver circuit 5302, the second scanline driver circuit 5303, and the signal line driver circuit 5304 areprovided over the same substrate 5300 as the pixel portion 5301.Therefore, since the number of components provided outside such as adriver circuit is reduced, it is possible not only to downsize thedisplay device but also to reduce cost due to decrease in the number ofassembly steps and inspection steps. Further, if the driver circuit isprovided outside the substrate 5300, wirings would need to be extendedand the number of connections of wirings would be increased, but byproviding the driver circuit over the substrate 5300, the number ofconnections of the wirings can be reduced. Therefore, decrease in yielddue to defective connection of the driver circuit and the pixel portioncan be prevented, and decrease in reliability due to low mechanicalstrength at a connection portion can be prevented.

Note that as an example, the timing control circuit 5305 supplies afirst scan line driver circuit start signal (GSP1) and a scan linedriver circuit clock signal (GCK1) to the first scan line driver circuit5302. Moreover, as an example, the timing control circuit 5305 suppliesa second scan line driver circuit start signal (GSP2) (also referred toas a start pulse) and a scan line driver circuit clock signal (GCK2) tothe second scan line driver circuit 5303. The timing control circuit5305 supplies a signal line driver circuit start signal (SSP), a signalline driver circuit clock signal (SCK), video signal data (DATA) (alsosimply referred to as a video signal) and a latch signal (LAT) to thesignal line driver circuit 5304. Note that each clock signal may be aplurality of clock signals whose periods are different or may besupplied together with an inverted clock signal (CKB). Either the firstscan line driver circuit 5302 or the second scan line driver circuit5303 can be omitted.

In FIG. 15B, a circuit with a low drive frequency (e.g., the first scanline driver circuit 5302 and the second scan line driver circuit 5303)is formed over the same substrate 5300 as the pixel portion 5301, andthe signal line driver circuit 5304 is formed over another substratewhich is different from the substrate provided with the pixel portion5301. It is also possible to form a circuit with a low drive frequencysuch as an analog switch used for a sampling circuit in the signal linedriver circuit 5304 partly over the same substrate 5300 as the pixelportion 5301. Thus, by partly employing system-on-panel, advantages ofsystem-on-panel such as the above-described prevention of decrease inyield due to defective connection, or low mechanical strength at aconnection portion, and reduction in cost due to decrease in the numberof assembly steps and inspection steps can be obtained more or less.Further, as compared with system-on-panel in which the pixel portion5301, the first scan line driver circuit 5302, the second scan linedriver circuit 5303, and the signal line driver circuit 5304 are formedover one substrate, by partly employing system-on-panel, it is possibleto increase performance of a circuit with a high drive frequency.Moreover, formation of a pixel portion having a large area is possible,which is difficult to realize in the case of using a single crystalsemiconductor.

Next, a structure of a signal line driver circuit including an n-channeltransistor is described.

The signal line driver circuit illustrated in FIG. 16A includes a shiftregister 5601 and a sampling circuit 5602. The sampling circuit 5602includes a plurality of switching circuits 5602_1 to 5602_N (N is anatural number). The switching circuits 5602_1 to 5602_N each include aplurality of n-channel transistors 5603_1 to 5603 _(—) k (k is a naturalnumber).

A connection relation in the signal line driver circuit is described byusing the switching circuit 5602_1 as an example. Note that one of asource electrode and a drain electrode included in a transistor isreferred to as a first terminal, and the other of the source electrodeand the drain electrode is referred to as a second terminal in thedescription below.

First terminals of the transistors 5603_1 to 5603 _(—) k are connectedto wirings 5604_1 to 5604 _(—) k, respectively. The video signal isinput to each of the wirings 5604_1 to 5604 _(—) k. Second terminals ofthe thin film transistors 5603_1 to 5603 _(—) k are connected to signallines S1 to Sk, respectively. Gate electrodes of the thin filmtransistors 5603_1 to 5603 _(—) k are connected to a wiring 5605_1.

The shift register 5601 has the function of sequentially selecting theswitching circuits 5602_1 to 5602_N by sequentially outputting timingsignals having a high voltage level (H level) to wirings 5605_1 to5605_N.

The switching circuit 5602_1 has a function of controlling a conductionstate between the wirings 5604_1 to 5604 _(—) k and the signal lines S1to Sk (a conduction state between the first terminal and the secondterminal), i.e., a function of controlling whether or not to supplypotentials of the wirings 5604_1 to 5604 _(—) k to the signal lines S1to Sk by switching of the transistors 5603_1 to 5603_N.

Next, operation of the signal line driver circuit shown in FIG. 16A isdescribed with reference to a timing chart in FIG. 16B. FIG. 16Billustrates a timing chart of timing signals Sout_1 to Sout_Nrespectively inputted to the wirings 5605_1 to 5605_N and video signalsVdata_1 to Vdata_k respectively inputted to the wirings 5604_1 to 5604_(—) k from the shift register 5601, as one example.

Note that one operation period of the signal line driver circuitcorresponds to one line period in a display device. FIG. 16B illustratesone example of the case where one line period is divided into periods T1to TN. Each of the periods T1 to TN is a period for writing a videosignal to one pixel belonging to the selected row.

In the periods T1 to TN, the shift register 5601 sequentially outputsH-level timing signals to the wirings 5605_1 to 5605_N. For example, inthe period T1, the shift register 5601 outputs an H-level signal to thewiring 5605_1. Then, the thin film transistors 5603_1 to 5603 _(—) kincluded in the switching circuit 5602_1 are turned on, so that thewirings 5604_1 to 5604 _(—) k and the signal lines S1 to Sk are broughtinto conduction. In this case, Data (S1) to Data (Sk) are input to thewirings 5604_1 to 5604 _(—) k, respectively. The Data (S1) to Data (Sk)are input to pixels in the first to k-th columns in the selected rowthrough the transistors 5603_1 to 5603 _(—) k. Thus, in the periods T1to TN, video signals are sequentially written to the pixels in theselected row by k columns.

By writing video signals to pixels of every plurality of columns, thenumber of video signals or the number of wirings can be reduced. Thus,connections to an external circuit such as a controller can be reduced.By writing video signals to pixels of every plurality of columns,writing time can be extended and insufficient writing of video signalscan be prevented.

Next, one mode of a shift register used for the signal line drivercircuit or the scan line driver circuit will be described with referenceto FIGS. 17A and 17B and FIGS. 18A and 18B.

The shift register includes first to N-th pulse output circuits 10_1 to10_N (N is a natural number of greater than or equal to 3) (see FIG.17A). A first clock signal CK1, a second clock signal CK2, a third clocksignal CK3, and a fourth clock signal CK4 are supplied from a firstwiring 11, a second wiring 12, a third wiring 13, and a fourth wiring14, respectively, to the first to Nth pulse output circuits 10_1 to10_N. A start pulse SP1 (a first start pulse) from a fifth wiring 15 isinput to the first pulse output circuit 10_1. A signal from a pulseoutput circuit of the previous stage (also referred to as a previousstage signal OUT (n−1) (n is a natural number of greater than or equalto 2) is input to the n-th pulse output circuit 10 _(—) n (n is anatural number of greater than or equal to 2 and less than or equal toN) of the second and subsequent stages. To the first pulse outputcircuit 10_1, a signal from the third pulse output circuit 10_3 of astage following the next stage is input. Similarly, to the n-th pulseoutput circuit 10 _(—) n of the second or subsequent stage, a signalfrom the (n+2)th pulse output circuit 10 _(— (n+)2) of the stagefollowing the next stage (such a signal is referred to as asubsequent-stage signal OUT(n+2)) is input. Therefore, from the pulseoutput circuits of the respective stages, first output signals(OUT(1)(SR) to OUT(N)(SR)) to be input to the pulse output circuits ofthe subsequent stages and/or the stages before the preceding stages andsecond output signals (OUT(1) to OUT(N)) to be input to differentcircuits or the like are output. Since later-stage signals OUT(n+2) arenot input to the pulse output circuits in the last two stages of theshift register, a structure in which a second start pulse SP2 and athird start pulse SP3 are input to the respective pulse output circuitsmay be employed, for example, as shown in FIG. 17A.

Note that a clock signal (CK) alternates between an H level and an Llevel (low level voltage) at regular intervals. The first to the fourthclock signals (CK1) to (CK4) are delayed by ¼ period sequentially. Inthis embodiment, by using the first to fourth clock signals (CK1) to(CK4), control or the like of driving of a pulse output circuit isperformed.

A first input terminal 21, a second input terminal 22, and a third inputterminal 23 are electrically connected to any of the first to fourthwirings 11 to 14. For example, in FIG. 17A, the first input terminal 21of the first pulse output circuit 10_1 is electrically connected to thefirst wiring 11, the second input terminal 22 of the first pulse outputcircuit 10_1 is electrically connected to the second wiring 12, and thethird input terminal 23 of the first pulse output circuit 10_1 iselectrically connected to the third wiring 13. In addition, the firstinput terminal 21 of the second pulse output circuit 10_2 iselectrically connected to the second wiring 12, the second inputterminal 22 of the second pulse output circuit 10_2 is electricallyconnected to the third wiring 13, and the third input terminal 23 of thesecond pulse output circuit 10_2 is electrically connected to the fourthwiring 14.

Each of the first to N-th pulse output circuits 10_1 to 10_N includesthe first input terminal 21, the second input terminal 22, the thirdinput terminal 23, a fourth input terminal 24, a fifth input terminal25, a first output terminal 26, and a second output terminal 27 (seeFIG. 17B). In the first pulse output circuit 10_1, the first clocksignal CK1 is input to the first input terminal 21; the second clocksignal CK2 is input to the second input terminal 22; the third clocksignal CK3 is input to the third input terminal 23; the start pulse isinput to the fourth input terminal 24; the next stage signal OUT (3) isinput to the fifth input terminal 25; the first output signal OUT (1)(SR) is output from the first output terminal 26; and the second outputsignal OUT (1) is output from the second output terminal 27.

Next, FIG. 18A illustrates one example of a specific circuit structureof a pulse output circuit.

The pulse output circuits each include first to thirteenth transistors31 to 43 (see FIG. 18A). Signals or power supply potentials are suppliedto the first to thirteenth transistors 31 to 43 from a power supply line51 which supplies a first high power supply potential VDD, a powersupply line 52 which supplies a second high power supply potential VCC,and a power supply line 53 which supplies a low power supply potentialVSS, in addition to the above-described first to fifth input terminals21 to 25, the first output terminal 26, and the second output terminal27. Here, the relation of the power supply potentials of the powersupply lines in FIG. 18A is as follows: a first power supply potentialVDD is higher than a second power supply potential VCC, and the secondpower supply potential VCC is higher than a third power supply potentialVSS. The first to fourth clock signals (CK1) to (CK4) alternate betweenH-level signals and L-level signals at regular intervals. The potentialis VDD when the clock signal is at the H level, and the potential is VSSwhen the clock signal is at the L level. By making the potential VDD ofthe power supply line 51 higher than the second power supply potentialVCC of the power supply line 52, a potential applied to a gate electrodeof a transistor can be lowered, shift in the threshold voltage of thetransistor can be reduced, and deterioration of the transistor can besuppressed without an adverse effect on the operation of the transistor.

In FIG. 18A, a first terminal of the first transistor 31 is electricallyconnected to the power supply line 51, a second terminal of the firsttransistor 31 is electrically connected to a first terminal of the ninthtransistor 39, and a gate electrode of the first transistor 31 iselectrically connected to the fourth input terminal 24. A first terminalof the second transistor 32 is electrically connected to the powersupply line 53, a second terminal of the second transistor 32 iselectrically connected to the first terminal of the ninth transistor 39,and a gate electrode of the second transistor 32 is electricallyconnected to a gate electrode of the fourth transistor 34. A firstterminal of the third transistor 33 is electrically connected to thefirst input terminal 21, and a second terminal of the third transistor33 is electrically connected to the first output terminal 26. A firstterminal of the fourth transistor 34 is electrically connected to thepower supply line 53, and a second terminal of the fourth transistor 34is electrically connected to the first output terminal 26. A firstterminal of the fifth transistor 35 is electrically connected to thepower supply line 53, a second terminal of the fifth transistor 35 iselectrically connected to the gate electrode of the second transistor 32and the gate electrode of the fourth transistor 34, and a gate electrodeof the fifth transistor 35 is electrically connected to the fourth inputterminal 24. A first terminal of the sixth transistor 36 is electricallyconnected to the power supply line 52, a second terminal of the sixthtransistor 36 is electrically connected to the gate electrode of thesecond transistor 32 and the gate electrode of the fourth transistor 34,and a gate electrode of the sixth transistor 36 is electricallyconnected to the fifth input terminal 25. A first terminal of theseventh transistor 37 is electrically connected to the power supply line52, a second terminal of the seventh transistor 37 is electricallyconnected to a second terminal of the eighth transistor 38, and a gateelectrode of the seventh transistor 37 is electrically connected to thethird input terminal 23. A first terminal of the eighth transistor 38 iselectrically connected to the gate electrode of the second transistor 32and the gate electrode of the fourth transistor 34, and a gate electrodeof the eighth transistor 38 is electrically connected to the secondinput terminal 22. The first terminal of the ninth transistor 39 iselectrically connected to the second terminal of the first transistor 31and the second terminal of the second transistor 32, a second terminalof the ninth transistor 39 is electrically connected to a gate electrodeof the third transistor 33 and a gate electrode of the tenth transistor40, and a gate electrode of the ninth transistor 39 is electricallyconnected to the power supply line 52. A first terminal of the tenthtransistor 40 is electrically connected to the first input terminal 21,a second terminal of the tenth transistor 40 is electrically connectedto the second output terminal 27, and the gate electrode of the tenthtransistor 40 is electrically connected to the second terminal of theninth transistor 39. A first terminal of the eleventh transistor 41 iselectrically connected to the power supply line 53, a second terminal ofthe eleventh transistor 41 is electrically connected to the secondoutput terminal 27, and the gate electrode of the eleventh transistor 41is electrically connected to the gate electrode of the second transistor32 and the gate electrode of the fourth transistor 34. A first terminalof the twelfth transistor 42 is electrically connected to the powersupply line 53, a second terminal of the twelfth transistor 42 iselectrically connected to the second output terminal 27, and a gateelectrode of the twelfth transistor 42 is electrically connected to thegate electrode of the seventh transistor 37. A first terminal of thethirteenth transistor 43 is electrically connected to the power supplyline 53, a second terminal of the thirteenth transistor 43 iselectrically connected to the first output terminal 26, and a gateelectrode of the thirteenth transistor 43 is electrically connected tothe gate electrode of the seventh transistor 37.

In FIG. 18A, a portion where the gate electrode of the third transistor33, the gate electrode of the tenth transistor 40, and the secondterminal of the ninth transistor 39 are connected is referred to as anode A. A portion where the gate electrode of the second transistor 32,the gate electrode of the fourth transistor 34, the second terminal ofthe fifth transistor 35, the second terminal of the sixth transistor 36,the first terminal of the eighth transistor 38, and the gate electrodeof the eleventh transistor 41 are connected is referred to as a node B(see FIG. 18A).

A timing chart of a shift register in which a plurality of pulse outputcircuits illustrated in FIG. 18A are provided is illustrated in FIG.18B.

Note that the placement of the ninth transistor 39 in which the secondpower supply potential VCC is applied to the gate electrode asillustrated in FIG. 18A has the following advantages before and afterbootstrap operation.

In the case where a potential of the node A is raised by bootstrapoperation without the provision of the ninth transistor 39 in which thesecond power supply potential VCC is applied to the gate electrode, apotential of the source electrode which is the second terminal of thefirst transistor 31 rises to a value higher than the first power supplypotential VDD. Then, the first terminal of the first transistor 31, thatis, the terminal on the power supply line 51 side, comes to serve as asource electrode of the first transistor 31. Consequently, in the firsttransistor 31, a high bias voltage is applied and thus significantstress is applied between the gate electrode and the source electrodeand between the gate electrode and the drain electrode, which mightcause deterioration of the transistor. By providing of the ninthtransistor 39 whose gate electrode is supplied with the second powersupply potential VCC, the potential of the node A is raised by thebootstrap operation, but at the same time, an increase in the potentialof the second terminal of the first transistor 31 can be prevented. Inother words, by providing of the ninth transistor 39, a negative biasvoltage applied between the gate electrode and the source electrode ofthe first transistor 31 can be reduced. Thus, the circuit configurationin this embodiment can reduce a negative bias voltage applied betweenthe gate electrode and the source electrode of the first transistor 31,so that deterioration of the first transistor 31 due to stress can besuppressed.

The place of the ninth transistor 39 is not limited as long as thesecond terminal of the first transistor 31 and the gate electrode of thethird transistor 33 are connected through the first terminal and thesecond terminal of the ninth transistor 39. Note that when the shiftregister including a plurality of pulse output circuits in thisembodiment is included in a signal line driver circuit having a largernumber of stages than a scan line driver circuit, the ninth transistor39 may be omitted, which is advantageous in that the number oftransistors is reduced.

Note that when oxide semiconductors are used for semiconductor layersfor the first to the thirteenth transistors 31 to 43, the off-current ofthe thin film transistors can be reduced, the on-current and the fieldeffect mobility can be increased, and the degree of deterioration can bereduced, whereby malfunction in a circuit can decrease. Further, thedegree of deterioration of the transistor using oxide semiconductorcaused by applying high potential to the gate electrode is small ascompared to the transistor using amorphous silicon. Therefore, even whenthe first power supply potential VDD is supplied to a power supply lineto which the second power supply potential VCC is supplied, a similaroperation can be performed, and the number of power supply lines whichare provided in a circuit can be reduced, so that the circuit can beminiaturized.

Note that a similar function is obtained even when the connectionrelation is changed so that a clock signal that is supplied to the gateelectrode of the seventh transistor 37 from the third input terminal 23and a clock signal that is supplied to the gate electrode of the eighthtransistor 38 from the second input terminal 22 are supplied from thesecond input terminal 22 and the third input terminal 23, respectively.In this case, in the shift register illustrated in FIG. 18A, the stateis changed from the state where both the seventh transistor 37 and theeighth transistor 38 are turned on, to the state where the seventhtransistor 37 is turned off and the eighth transistor 38 is turned on,and then to the state where both the seventh transistor 37 and theeighth transistor 38 are turned off; thus, the fall in a potential ofthe node B due to fall in the potentials of the second input terminal 22and the third input terminal 23 is caused twice by fall in the potentialof the gate electrode of the seventh transistor 37 and fall in thepotential of the gate electrode of the eighth transistor 38. On thecontrary, in the shift register shown in FIG. 18A is driven so that thestate where the seventh transistor 37 and the eighth transistor 38 areboth on is changed through the state where the seventh transistor 37 ison and the eighth transistor 38 is off to the state where the seventhtransistor 37 is off and the eighth transistor 38 is off, potentialreduction at the node B, which is caused by fall in the of the secondinput terminal 22 and the third input terminal 23, is caused only oncedue to fall in the potential of the gate electrode of the eighthtransistor 38. Therefore, the connection relation, that is, the clocksignal is supplied from the third input terminal 23 to the gateelectrode of the seventh transistor 37 and the clock signal is suppliedfrom the second input terminal 22 to the gate electrode of the eighthtransistor 38, is preferable. That is because the number of times of thechange in the potential of the node B can be reduced, whereby the noisecan be reduced.

In this manner, in a period during which the potentials of the firstoutput terminal 26 and the second output terminal 27 are held at the Llevel, the H level signal is regularly supplied to the node B;therefore, malfunction of a pulse output circuit can be suppressed.

This embodiment can be implemented in combination with any of the aboveembodiments.

Embodiment 7

In this embodiment, manufacturing methods of semiconductor displaydevices according to one embodiment of the present invention aredescribed with reference to FIGS. 19A to 19C, FIGS. 20A to 20C, FIGS.21A and 21B, FIG. 22, FIG. 23, and FIG. 24.

Note that the term “successive film formation” in this specificationmeans that during a series of a first film formation step by sputteringand a second film formation step by sputtering, an atmosphere in which asubstrate to be processed is disposed is not contaminated by acontaminant atmosphere such as air, and is constantly controlled to bevacuum or an inert gas atmosphere (a nitrogen atmosphere or a rare gasatmosphere). By the successive film formation, film formation can beconducted to a substrate which has been cleaned, without re-attachmentof moisture or the like.

Performing the process from the first film formation step to the secondfilm formation step in the same chamber is within the scope of thesuccessive formation in this specification.

In addition, the following is also within the scope of the successiveformation in this specification: in the case of performing the processfrom the first film formation step to the second film formation step inplural chambers, the substrate is transferred after the first filmformation step to another chamber without being exposed to air andsubjected to the second film formation.

Note that between the first film formation step and the second filmformation step, a substrate transfer step, an alignment step, aslow-cooling step, a step of heating or cooling the substrate to atemperature which is necessary for the second film formation step, orthe like may be provided. Such a process is also within the scope of thesuccessive formation in this specification.

A step in which liquid is used, such as a cleaning step, wet etching, orformation of a resist may be provided between the first deposition stepand the second deposition step. This case is not within the scope of thesuccessive deposition in this specification.

In FIG. 19A, a light-transmitting substrate 400 may be a glass substratemanufactured by a fusion method or a float method, or a metal substrateformed of a stainless alloy having an insulating film on the surface. Asubstrate formed from a flexible synthetic resin, such as plastic,generally tends to have a low allowable temperature limit, but can beused as the substrate 400 as long as the substrate can withstandprocessing temperatures in the later manufacturing process. Examples ofa plastic substrate include polyester typified by polyethyleneterephthalate (PET), polyethersulfone (PES), polyethylene naphthalate(PEN), polycarbonate (PC), polyetheretherketone (PEEK), polysulfone(PSF), polyetherimide (PEI), polyarylate (PAR), polybutyleneterephthalate (PBT), polyimide, acrylonitrile-butadiene-styrene resin,polyvinyl chloride, polypropylene, polyvinyl acetate, acrylic resin, andthe like.

In the case where a glass substrate is used and the temperature at whichthe heat treatment is to be performed later is high, a glass substratewhose strain point is more than or equal to 730° C. is preferably used.As a glass substrate, a glass material such as aluminosilicate glass,aluminoborosilicate glass, or barium borosilicate glass is used, forexample. In general, a glass substrate containing more barium oxide(BaO) than diboron trioxide (B₂O₃) is more practical as heat-resistantglass. Therefore, a glass substrate containing a larger amount of BaOthan B₂O₃ is preferably used.

Note that as the above glass substrate, a substrate formed of aninsulator such as a ceramic substrate, a quartz substrate, or a sapphiresubstrate may be used. Alternatively, crystallized glass or the like maybe used.

Next, a conductive film is formed entirely over a surface of thesubstrate 400, and then a first photolithography step is performed insuch a manner that a resist mask is formed and unnecessary portions areremoved by etching, so that wirings and an electrode (a gate wiringincluding a gate electrode 401, a capacitor wiring 408, and a firstterminal 421) are formed. At this time, the etching is performed so thatat least end portions of the gate electrode 401 are tapered.

A material for the conductive film can be a single layer or a stackedlayer using one or more of a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, neodymium, or scandium, or analloy material which contains any of these metal materials as a maincomponent, or nitride of these metals. Note that aluminum or copper canalso be used as the above metal material as long as it can withstand atemperature of heat treatment performed in a later step.

For example, as a conductive film having a two-layer stack structure,the following structures are preferable: a two-layer structure of analuminum layer and a molybdenum layer stacked thereover, a two-layerstructure of a copper layer and a molybdenum layer stacked thereover, atwo-layer structure of a copper layer and a titanium nitride layer or atantalum nitride layer stacked thereover, and a two-layer structure of atitanium nitride layer and a molybdenum layer. As a three-layerstructure, the following structure is preferable: a layered structurecontaining aluminum, an alloy of aluminum and silicon, an alloy ofaluminum and titanium, or an alloy of aluminum and neodymium in a middlelayer and any of tungsten, tungsten nitride, titanium nitride, andtitanium in a top layer and a bottom layer.

A light-transmitting oxide conductive layer can be used for part of theelectrode layer and the wiring to increase the aperture ratio. Forexample, indium oxide, an alloy of indium oxide and tin oxide, an alloyof indium oxide and zinc oxide, zinc oxide, zinc aluminum oxide, zincaluminum oxynitride, zinc gallium oxide, or the like can be used.

The thicknesses of the gate electrode 401, the capacitor wiring 408, andthe first terminal 421 are each 10 nm to 400 nm, preferably 100 nm to200 nm. In this embodiment, after a conductive film with a thickness of100 nm for the gate electrode is formed by a sputtering method using atungsten target, the conductive film is processed (patterned) by etchingto have a desired shape, so that the gate electrode 401, the capacitorwiring 408, and the first terminal 421 are formed.

An insulating film serving as a base film may be provided between thesubstrate 400 and the gate electrode 401, the capacitor wiring 408, andthe first terminal 421. The base film has a function of preventingdiffusion of an impurity element from the substrate 400, and can beformed with a single layer or stacked layer using one or more filmsselected from a silicon nitride film, a silicon oxide film, a siliconnitride oxide film, and a silicon oxynitride film.

Next, a gate insulating film 402 is formed entirely over surfaces of thegate electrode 401, the capacitor wiring 408, the first terminal 421 asillustrated in FIG. 19B. The gate insulating film 402 can be formed tohave a single layer of a silicon oxide film, a silicon nitride film, asilicon oxynitride film, a silicon nitride oxide film, an aluminum oxidefilm, or a tantalum oxide film or a stacked layer thereof by a plasmaCVD method, a sputtering method, or the like. For example, a siliconoxynitride film may be formed using a deposition gas including silane(for example, monosilane), oxygen, and nitrogen by a plasma CVD method.

The film thickness of the gate insulating film 402 is desirably morethan or equal to 50 nm and less than or equal to 250 nm. In thisembodiment, a silicon oxynitride film with a thickness of 100 nm formedby a plasma CVD method is used as the gate insulating film 402.

Next, an oxide semiconductor film 403 is formed over the gate insulatingfilm 402. The oxide semiconductor film 403 is formed by a sputteringmethod with use of an oxide semiconductor as a target. Moreover, theoxide semiconductor film 403 can be formed by a sputtering method undera rare gas (for example, argon) atmosphere, an oxygen atmosphere, or anatmosphere including a rare gas (for example, argon) and oxygen.

It is preferable that before the oxide semiconductor film 403 is formedby a sputtering method, dust on a surface of the gate insulating film402 be removed by reverse sputtering by introducing an argon gas andgenerating plasma. The reverse sputtering refers to a method in which,without application of a voltage to a target side, an RF power source isused for application of a voltage to a substrate side in an argonatmosphere to generate plasma in the vicinity of the substrate to modifya surface. Note that instead of an argon atmosphere, a nitrogenatmosphere, a helium atmosphere, or the like may be used. Alternatively,an argon atmosphere to which oxygen, nitrous oxide, or the like is addedmay be used. Alternatively, an argon atmosphere to which chlorine,carbon tetrafluoride, or the like is added may be used.

The oxide semiconductor film 403 for formation of a channel formationregion may be formed using the above-described oxide material havingsemiconductor characteristics.

The thickness of the oxide semiconductor film 403 is 5 nm to 300 nm,preferably 10 nm to 100 nm. In this embodiment, film deposition isperformed using an oxide semiconductor target containing In, Ga, and Zn(In₂O₃:Ga₂O₃:ZnO=1:1:1 or In₂O₃:Ga₂O₃:ZnO=1:1:2 [mol ratio]) under thefollowing condition: the distance between a substrate and a target is100 mm, the pressure is 0.6 Pa, the direct-current (DC) power supply is0.5 kW, and the atmosphere is oxygen (the flow rate of oxygen is 100%).Note that a pulse direct current (DC) power supply is preferable becausedust due to film deposition can be reduced and the film thickness can beuniform. In this embodiment, a 50 nm-thick In—Ga—Zn—O-basednon-single-crystal film is formed as the oxide semiconductor film.

After the sputtering, the oxide semiconductor film is formed withoutexposure to the air, whereby adhesion of dust and moisture to aninterface between the gate insulating film 402 and the oxidesemiconductor film 403 can be prevented. Further, a pulsed directcurrent (DC) power supply is preferable because dust can be reduced anda thickness distribution is uniform.

It is preferable that the relative density of the oxide semiconductortarget is greater than or equal to 80%, more preferably greater than orequal to 95%, further preferably, greater than or equal to 99.9%. Theimpurity concentration in the oxide semiconductor film which is formedusing the target having high relative density can be reduced, and thus athin film transistor having high electric characteristics or highreliability can be obtained.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

In addition, there are a sputtering apparatus provided with a magnetsystem inside the chamber and used for a magnetron sputtering, and asputtering apparatus used for an ECR sputtering in which plasmagenerated with the use of microwaves is used without using glowdischarge.

Furthermore, as a deposition method by sputtering, there are also areactive sputtering method in which a target substance and a sputteringgas component are chemically reacted with each other during depositionto form a thin compound film thereof, and a bias sputtering in which avoltage is also applied to a substrate during deposition.

In addition, the substrate may be heated at a temperature of more thanor equal to 400° C. and less than or equal to 700° C. by light or aheater during the film formation with a sputtering method. The damagedue to sputtering is repaired at the same time as the film formation byheating during the film formation.

Preheat treatment is preferably performed so as to remove moisture orhydrogen remaining on an inner wall of the sputtering apparatus, on asurface of the target, or in a target material, before the oxidesemiconductor film is formed. As the preheat treatment, a method inwhich the inside of the film formation chamber is heated to 200° C. to600° C. under reduced pressure, a method in which introduction andevacuation of nitrogen or an inert gas are repeated while the inside ofthe film formation chamber is heated, and the like can be given. Afterthe preheat treatment, the substrate or the sputtering apparatus iscooled, and then the oxide semiconductor film is formed without exposureto air. In this case, not water but oil or the like is preferably usedas a coolant for the target. Although a certain level of effect can beobtained when introduction and evacuation of nitrogen are repeatedwithout heating, it is more preferable to perform the treatment with theinside of the film formation chamber heated.

It is preferable to remove moisture or the like remaining in thesputtering apparatus with the use of a cryopump before, during, or afterthe oxide semiconductor film is formed.

Next, as illustrated in FIG. 19C, a second photolithography step isperformed in such a manner that a resist mask is formed and the oxidesemiconductor film 403 is etched. For example, unnecessary portions areremoved by wet etching using a mixed solution of phosphoric acid, aceticacid, and nitric acid, so that an island-shaped oxide semiconductor film404 can be formed so as to overlap with the gate electrode 401. Inetching of the oxide semiconductor film 403, organic acid such as citricacid or oxalic acid can be used for etchant. In this embodiment, theunnecessary portions are removed by wet etching using ITO07N (product ofKanto Chemical Co., Inc.), so that the island-shaped oxide semiconductorfilm 404 is formed. Note that etching here is not limited to wet etchingand dry etching may be used.

As the etching gas for dry etching, a gas containing chlorine(chlorine-based gas such as chlorine (Cl₂), boron chloride (BCl₃),silicon chloride (SiCl₄), or carbon tetrachloride (CCl₄)) is preferablyused.

Alternatively, a gas containing fluorine (fluorine-based gas such ascarbon tetrafluoride (CF₄), sulfur fluoride (SF₆), nitrogen fluoride(NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr); oxygen (O₂);any of these gases to which a rare gas such as helium (He) or argon (Ar)is added; or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. In order to etch the films into desired shapes, the etchingcondition (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) is adjusted as appropriate.

The etchant after the wet etching is removed together with the etchedmaterials by cleaning. The waste liquid including the etchant and thematerial etched off may be purified and the material may be reused. Whena material such as indium contained in the oxide semiconductor film iscollected from the waste liquid after the etching and reused, theresources can be efficiently used and the cost can be reduced.

In order to obtain a desired shape by etching, the etching conditions(such as an etchant, etching time, and temperature) are adjusted asappropriate depending on the material.

Next, as illustrated in FIG. 20A, heat treatment may be performed on theoxide semiconductor film 404 under a reduced-pressure atmosphere, anatmosphere of an inert gas such as nitrogen and a rare gas, an oxygengas atmosphere, or an ultra-dry air atmosphere (the moisture amount is20 ppm (−55° C. by conversion into a dew point) or less, preferably 1ppm or less, more preferably 10 ppb or less when measured by a dew pointmeter in a CRDS (cavity ring down laser spectroscopy) method). With theheat treatment on the oxide semiconductor film 404, the oxidesemiconductor film 405 is formed. Specifically, under an inert gasatmosphere (e.g., nitrogen, helium, neon, or argon), rapid thermalannealing (RTA) treatment can be performed at a temperature of more thanor equal to 500° C. and less than or equal to 750° C. (or a temperaturelower than or equal to the strain point of the glass substrate) forapproximately more than or equal to 1 minute and less than or equal to10 minutes, preferably, at 650° C. for approximately more than or equalto 3 minutes and less than or equal to 6 minutes. With an RTA method,dehydration or dehydrogenation can be performed in a short time;therefore, treatment can be performed even at a temperature higher thanthe strain point of the glass substrate. Note that the timing of theabove-described heat treatment is not limited to this timing afterformation of the oxide semiconductor film 404, and the oxidesemiconductor film 403 before formation of the oxide semiconductor film404 may be subjected to the heat treatment. The heat treatment may alsobe performed plural times after formation of the oxide semiconductorfilm 404.

Further, a heating method using an electric furnace, a rapid heatingmethod such as a gas rapid thermal annealing (GRTA) method using aheated gas or a lamp rapid thermal annealing (LRTA) method using lamplight, or the like can be used for the heat treatment. For example, inthe case of performing heat treatment using an electric furnace, thetemperature rise characteristics is preferably set at higher than orequal to 0.1° C./min and lower than or equal to 20° C./min and thetemperature drop characteristics is preferably set at higher than orequal to 0.1° C./min and lower than or equal to 15° C./min.

Note that in heat treatment, it is preferable that moisture, hydrogen,and the like be not contained in nitrogen or a rare gas such as helium,neon, or argon. It is preferable that the purity of nitrogen or the raregas such as helium, neon, or argon which is introduced into a heattreatment apparatus be set to be 6N (99.9999%) or higher, preferably 7N(99.99999%) or higher (that is, the impurity concentration is 1 ppm orlower, preferably 0.1 ppm or lower).

After the heat treatment under an inert gas atmosphere, theisland-shaped oxide semiconductor film 405 may be crystallized partly orentirely.

Cross-sectional views taken along dashed lines C1-C2 and D1-D2 in FIG.20A correspond to cross-sectional views taken along dashed lines C1-C2and D1-D2 in a plan view illustrated in FIG. 22, respectively.

Next, as illustrated in FIG. 20B, a conductive film 406 is formed usinga metal material over the oxide semiconductor film 405 by a sputteringmethod or a vacuum evaporation method. As a material of a conductivefilm 406, for example, an element selected from titanium, tungsten, andmolybdenum, an alloy containing one or more of the above elements, orthe like can be used. In a semiconductor device of one embodiment of thepresent invention, in the source electrode 407 a and the drain electrode407 b, at least a portion which is the closest to the island-shapedoxide semiconductor film 405 may be formed using an element selectedfrom titanium, tungsten, and molybdenum, an alloy containing one or moreof the above elements, or the like. Therefore, in the case where thesource electrode 407 a and the drain electrode 407 b each having astructure in which a plurality of metal films are stacked, a metal filmthat is in contact with the oxide semiconductor film 405 may be formedusing titanium, tungsten, or molybdenum, and the other metal films canbe formed using any of the following examples: an element selected fromaluminum, chromium, tantalum, titanium, manganese, magnesium,molybdenum, tungsten, zirconium, beryllium, and yttrium; an alloycontaining one or more of the above elements as a component; a nitridecontaining the above element as a component; or the like. For example,by using a conductive film 406 having a stacked structure of a titaniumfilm, an aluminum alloy film containing neodymium, and a titanium film,and by using the titanium film in the portion which is the closest tothe island-shaped oxide semiconductor film 405, the source electrode 407a and the drain electrode 407 b can have a low resistance and high heatresistance in the aluminum alloy film containing neodymium.

Note that in the case where heat treatment is performed after theformation of the conductive film 406 for the source electrode and thedrain electrode, the conductive film 406 preferably has heat resistanceenough to withstand the heat treatment. In the case of performing heattreatment after the formation of the conductive film 406, the conductivefilm 406 is formed in combination with the heat-resistant conductivematerial because aluminum alone has problems of low heat resistance,being easily corroded, and the like. As the heat-resistant conductivematerial which is combined with aluminum, the following material ispreferably used: an element selected from titanium, tantalum, tungsten,molybdenum, chromium, neodymium, and scandium; an alloy containing oneor more of these elements as a component; a nitride containing any ofthese elements as a component; or the like.

The thickness of the conductive film 406 for the source electrode andthe drain electrode is 10 nm to 400 nm, preferably 100 nm to 200 nm. Inthis embodiment, the conductive film 406 for a source electrode and adrain electrode is formed by a sputtering method using a titaniumtarget.

By forming the conductive film 406 having the above structure, oxygen inthe region of the oxide semiconductor film 405 which is the closest tothe conductive film 406 is taken out, so that the composite layers 430where the concentration of a metal contained in the oxide semiconductorfilm 405 is higher than that in other regions (metal-rich layers) areformed in the oxide semiconductor film 405. The oxygen that is taken outreacts with the metal in the conductive film 406, so that the metaloxide films 431 are formed between the conductive film 406 and themetal-rich composite layers 430.

Next, as illustrated in FIG. 20C, a third photolithography step isperformed in such a manner that a resist mask is formed and unnecessaryportions of the conductive film 406 are removed by wet etching or dryetching, so that a source electrode 407 a, a drain electrode 407 b, anda second terminal 420 are formed. For example, in the case where theconductive film 406 is formed using titanium, wet etching can beperformed by using a hydrogen peroxide solution or heated hydrochloricacid as etchant. Note that since oxygen is further taken out from theoxide semiconductor film 412 by the heat treatment, it is possible toincrease the thickness of the composite layers 430 and the metal oxidefilms 431.

In the above-described etching step, since the composite layer 430 isetched in an exposed region of the oxide semiconductor film 405, anisland-shaped oxide semiconductor film 409 having a thin region betweenthe source electrode 407 a and the drain electrode 407 b can be formedin some cases.

In addition, in the above-described etching, the metal oxide film 431 isetched together with the conductive film 406. Thus, there are the etchedmetal oxide film 431 between the composite layer 430 of the oxidesemiconductor film 409 and the source electrode 407 a, and the etchedmetal oxide film 431 between the composite layer 430 of the oxidesemiconductor film 409 and the drain electrode 407 b. The compositelayer 430 on the source electrode 407 a side and the composite layer 430on the drain electrode 407 b side are separated from each other. Inaddition, the metal oxide film 431 on the source electrode 407 a sideand the metal oxide film 431 on the drain electrode 407 b side areseparated from each other.

For example, in the case where an In—Ga—Zn—O-based oxide semiconductoris used for the oxide semiconductor film 405, the composite layers 430where the concentration of indium is higher than that in other regions(In-rich layers) exist in regions of the oxide semiconductor film 405which are the closest to the source electrode 407 a and the drainelectrode 407 b, so that resistance of the In-rich composite layers 430in the oxide semiconductor film 405 becomes lower. In the case wheretitanium is used for the source electrode 407 a and the drain electrode407 b, the metal oxide films 431 formed between the source electrode 407a and the oxide semiconductor film 405, and between the drain electrode407 b and the oxide semiconductor film 405 contain titanium oxide (TiOx)and have n-type conductivity. Therefore, with the above structure,contact resistance between the source electrode 407 a and the oxidesemiconductor film 405, and between the drain electrode 407 b and theoxide semiconductor film 405 is reduced, and the amount of on-currentand field effect mobility of a TFT can be increased.

In the third photolithography step, the second terminal 420 which isformed using the same material as the source electrode 407 a and thedrain electrode 407 b is left in the terminal portion. Note that thesecond terminal 420 is electrically connected to a source wiring (asource wiring including the source electrode 407 a and the drainelectrode 407 b).

Further, by using a resist mask which is formed using a multi-tone maskand has regions with plural thicknesses (for example, two differentthicknesses), the number of resist masks can be reduced, resulting insimplified process and lower costs.

Cross-sectional views taken along dashed lines C1-C2 and D1-D2 in FIG.20C correspond to cross-sectional views taken along dashed lines C1-C2and D1-D2 in a plan view illustrated in FIG. 23, respectively.

Next, as illustrated in FIG. 21A, an oxide insulating film 411 whichcovers the gate insulating film 402, the oxide semiconductor film 409,the source electrode 407 a, and the drain electrode 407 b is formed. Inthis embodiment, a silicon oxide film with a thickness of 300 nm isformed as the oxide insulating film 411. The substrate temperature infilm formation may be higher than or equal to room temperature and lowerthan or equal to 300° C. and is 100° C. in this embodiment. Formation ofthe silicon oxide film with a sputtering method can be performed under arare gas (for example, argon) atmosphere, an oxygen atmosphere, or anatmosphere including a rare gas (for example, argon) and oxygen.Further, a silicon oxide target or a silicon target may be used as atarget. For example, with use of a silicon target, a silicon oxide filmcan be formed by a sputtering method under an atmosphere of oxygen andnitrogen.

By providing the oxide insulating film 411 in contact with the exposedregion of the oxide semiconductor film 409 provided between the sourceelectrode 407 a and the drain electrode 407 b, the resistance of theregion of the oxide semiconductor film 409 which is in contact with theoxide insulating film 411 becomes higher (the carrier concentration isdecreased, preferably to a value lower than 1×10¹⁸/cm³), resulting information of an oxide semiconductor film 412 having a high-resistancechannel formation region.

In this embodiment, the oxide insulating film 411 with a thickness of300 nm is formed by a pulsed DC sputtering method using a columnarpolycrystalline, boron-doped silicon target which has a purity of 6N(the resistivity is 0.01 Ωcm), in which the distance between thesubstrate and the target (T-S distance) is 89 mm, the pressure is 0.4Pa, the direct-current (DC) power source is 6 kW, and the atmosphere isoxygen (the oxygen flow rate is 100%).

Next, after the oxide insulating film 411 is formed, second heattreatment may be performed. The second heat treatment is performed undera reduced-pressure atmosphere, an atmosphere of an inert gas such asnitrogen and a rare gas, an oxygen gas atmosphere, or an ultra-dry airatmosphere (the moisture amount is 20 ppm (−55° C. by conversion into adew point) or less, preferably 1 ppm or less, more preferably 10 ppb orless when measured by a dew point meter in a CRDS (cavity ring downlaser spectroscopy) method, at a temperature of more than or equal to200° C. and less than or equal to 400° C., for example, more than orequal to 250° C. and less than or equal to 350° C.). For example, thesecond heat treatment is performed in a nitrogen atmosphere at 250° C.for one hour. Alternatively, RTA treatment may be performed at hightemperature for a short time as in the previous heat treatment. By theheat treatment, the oxide semiconductor film 412 is heated while beingin contact with the oxide insulating film 411. In addition, theresistance of the oxide semiconductor film 412 is increased.Accordingly, electric characteristics of the transistor can be improvedand variation in the electric characteristics thereof can be reduced.There is no particular limitation on when to perform this heat treatmentas long as it is performed after the formation of the oxide insulatingfilm 411. When this heat treatment also serves as heat treatment inanother step, for example, heat treatment in formation of a resin filmor heat treatment for reducing resistance of a transparent conductivefilm, the number of steps can be prevented from increasing.

Through the above steps, a thin film transistor 413 can be manufactured.

Next, a fourth photolithography step is performed in such a manner thata resist mask is formed and the oxide insulating film 411 and the gateinsulating film 402 are etched, so that a contact hole is formed toexpose parts of the drain electrode 407 b, the first terminal 421, andthe second terminal 420. Next, the resist mask is removed, and then atransparent conductive film is formed. The transparent conductive filmis formed of indium oxide (In₂O₃), indium oxide-tin oxide alloy(In₂O₃—SnO₂, abbreviated to ITO), or the like by a sputtering method, avacuum evaporation method, or the like. Such a material is etched with ahydrochloric acid-based solution. However, since a residue is easilygenerated particularly in etching ITO, indium oxide-zinc oxide alloy(In₂O₃—ZnO) may be used to improve etching processability. Moreover, inthe case where heat treatment for reducing resistance of the transparentconductive film, the heat treatment can serve as heat treatment forincreasing resistance of the oxide semiconductor film 412, which resultsin improvement of electric characteristics of the transistor andreduction in variation in the electric characteristics thereof.

Next, a fifth photolithography step is performed in such a manner that aresist mask is formed and unnecessary portions are removed by etching,so that a pixel electrode 414 which is connected to the drain electrode407 b, a transparent conductive film 415 which is connected to the firstterminal 421, and a transparent conductive film 416 which is connectedto the second terminal 420 are formed.

The transparent conductive films 415 and 416 serve as electrodes orwirings connected to an FPC. The transparent conductive film 415 formedover the first terminal 421 is a connection terminal electrode whichfunctions as an input terminal of the gate wiring. The transparentconductive film 416 formed over the second terminal 420 is a connectionterminal electrode which functions as an input terminal of the sourcewiring.

In the fifth photolithography step, a storage capacitor is formed withthe capacitor wiring 408 and the pixel electrode 414, in which the gateinsulating film 402 and the oxide insulating film 411 are used asdielectrics.

A cross-sectional view after the resist mask is removed is illustratedin FIG. 21B. Cross-sectional views taken along dashed lines C1-C2 andD1-D2 in FIG. 21B correspond to cross-sectional views taken along dashedlines C1-C2 and D1-D2 in a plan view illustrated in FIG. 24,respectively.

Through these six photolithography steps, the storage capacitor and thethin film transistor 413 which is a bottom-gate staggered thin filmtransistor can be completed using the six photomasks. By disposing thethin film transistor and the storage capacitor in each pixel of a pixelportion in which pixels are arranged in a matrix form, one of substratesfor manufacturing an active matrix display device can be obtained. Inthis specification, such a substrate is referred to as an active matrixsubstrate for convenience.

In the case of manufacturing an active matrix liquid crystal displaydevice, an active matrix substrate and a counter substrate provided witha counter electrode are bonded to each other with a liquid crystal layerinterposed therebetween.

Alternatively, a storage capacitor may be formed with a pixel electrodewhich overlaps with a gate wiring of an adjacent pixel, with an oxideinsulating film and a gate insulating film interposed therebetween,without provision of the capacitor wiring.

In an active matrix liquid crystal display device, pixel electrodesarranged in a matrix form are driven to form a display pattern on ascreen. Specifically, a voltage is applied between a selected pixelelectrode and a counter electrode corresponding to the pixel electrode,so that a liquid crystal layer provided between the pixel electrode andthe counter electrode is optically modulated and this optical modulationis recognized as a display pattern by an observer.

In manufacturing a light-emitting display device, a partition wallincluding an organic resin film is provided between organiclight-emitting elements in some cases. In that case, heat treatmentperformed on the organic resin film can also serve as the heat treatmentwhich increases the resistance of the oxide semiconductor film 412 sothat improvement and less variation in electric characteristics of thetransistor are achieved.

The use of an oxide semiconductor for a thin film transistor leads toreduction in manufacturing cost. In particular, by the heat treatment,impurities such as moisture, hydrogen, or OH are reduced and the purityof the oxide semiconductor film is increased. As a result, asemiconductor display device including a highly reliable thin filmtransistor having favorable electric characteristics can bemanufactured.

Since the semiconductor film in the channel formation region is a regionwhose resistance is increased, electric characteristics of the thin filmtransistor are stabilized, and increase in off-current or the like canbe prevented. Accordingly, a semiconductor display device including thehighly reliable thin film transistor having favorable electriccharacteristics can be provided.

This embodiment can be implemented in combination with any of the aboveembodiments.

Embodiment 8

In the liquid crystal display device according to one embodiment of thepresent invention, a highly reliable thin film transistor with highmobility and on-current is used; therefore, the liquid crystal displaydevice according to one embodiment of the present invention has highcontrast and high visibility. In this embodiment, a structure of theliquid crystal display device according to an embodiment of the presentinvention is described.

FIG. 25 illustrates as an example a cross-sectional view of a pixel in aliquid crystal display device of one embodiment of the presentinvention. The thin film transistor 1401 illustrated in FIG. 25 includesa gate electrode 1402 formed over an insulating surface, a gateinsulating film 1403 over the gate electrode 1402, an oxidesemiconductor film 1404 which overlaps with the gate electrode 1402 overthe gate insulating film 1403 and which includes composite layers 1420where the concentration of one or a plurality of metals contained in theoxide semiconductor is higher than that in other regions, a pair ofmetal oxide films 1421 formed over the oxide semiconductor film 1404 andin contact with the composite layers 1420, and a pair of conductivefilms 1406 which function as a source electrode and a drain electrodeand which are in contact with the metal oxide films 1421. Further, thethin film transistor 1401 may include as its component an oxideinsulating film 1407 formed over the oxide semiconductor film 1404. Theoxide insulating film 1407 is formed so as to cover the gate electrode1402, the gate insulating film 1403, the oxide semiconductor film 1404,and the pair of conductive films 1406. The metal oxide films 1421 areformed by oxidation of a metal contained in the pair of conductive films1406.

An insulating film 1408 is formed over the oxide insulating film 1407.An opening is provided in part of the oxide insulating film 1407 and theinsulating film 1408, and a pixel electrode 1410 is formed so as to bein contact with one of the conductive films 1406 in the opening.

Further, a spacer 1417 for controlling a cell gap of a liquid crystalelement is formed over the insulating film 1408. An insulating film isetched to have a desired shape, so that the spacer 1417 can be formed. Acell gap may also be controlled by dispersing a filler over theinsulating film 1408.

An alignment film 1411 is formed over the pixel electrode 1410. Thealignment film 1411 can be formed by subjecting an insulating film torubbing treatment. Further, a counter electrode 1413 is provided in aposition opposed to the pixel electrode 1410, and an alignment film 1414is formed on the side of the counter electrode 1413 which is close tothe pixel electrode 1410. Furthermore, a liquid crystal 1415 is providedin a region which is surrounded by a sealant 1416 between the pixelelectrode 1410 and the counter electrode 1413. Note that a filler may bemixed in the sealant 1416.

The pixel electrode 1410 and the counter electrode 1413 can be formedusing a transparent conductive material such as indium tin oxidecontaining silicon oxide (ITSO), indium tin oxide (ITO), zinc oxide(ZnO), indium zinc oxide (IZO), or gallium-doped zinc oxide (GZO), forexample. Note that this embodiment shows an example of manufacturing atransmissive type liquid crystal element by using a light-transmittingconductive film for the pixel electrode 1410 and the counter electrode1413. The liquid crystal display device according to an embodiment ofthe present invention may be a semi-transmissive type liquid crystaldisplay device or a reflective type liquid crystal display device.

The liquid crystal display device illustrated in FIG. 25 may be providedwith a color filter, a shielding film for preventing disclination (ablack matrix), or the like.

Although a liquid crystal display device of a TN (twisted nematic) modeis described in this embodiment, the thin film transistor of oneembodiment of the present invention can be used for other liquid crystaldisplay devices of a VA (vertical alignment) mode, an OCB (opticallycompensated birefringence) mode, an IPS (in-plane-switching) mode, andthe like.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated right before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is increased. Since the blue phase is generated within an onlynarrow range of temperature, liquid crystal composition in which achiral agent at 5 wt % or more is mixed is used for the liquid crystal1415 in order to improve the temperature range. The liquid crystalcomposition which includes liquid crystal exhibiting a blue phase and achiral agent have such characteristics that the response time is morethan or equal to 10 μsec. and less than or equal to 100 μsec., which isshort, the alignment process is unnecessary because the liquid crystalcomposition has optical isotropy, and viewing angle dependency is small.

FIG. 27 illustrates an example of a perspective view showing a structureof a liquid crystal display device of the present invention. The liquidcrystal display device shown in FIG. 27 is provided with a liquidcrystal panel 1601 in which a liquid crystal element is formed between apair of substrates; a first diffusing plate 1602; a prism sheet 1603; asecond diffusing plate 1604; a light guide plate 1605; a reflectionplate 1606; a light source 1607; and a circuit substrate 1608.

The liquid crystal panel 1601, the first diffusing plate 1602, the prismsheet 1603, the second diffusing plate 1604, the light guide plate 1605,and the reflection plate 1606 are stacked in this order. The lightsource 1607 is provided at an end portion of the light guide plate 1605.The liquid crystal panel 1601 is uniformly irradiated with light fromthe light source 1607 which is diffused inside the light guide plate1605, due to the first diffusing plate 1602, the prism sheet 1603, andthe second diffusing plate 1604.

Although the first diffusing plate 1602 and the second diffusing plate1604 are used in this embodiment, the number of diffusing plates is notlimited thereto. The number of diffusing plates may be one, or may bethree or more. It is acceptable as long as the diffusing plate isprovided between the light guide plate 1605 and the liquid crystal panel1601. Therefore, a diffusing plate may be provided only on the sidecloser to the liquid crystal panel 1601 than the prism sheet 1603, ormay be provided only on the side closer to the light guide plate 1605than the prism sheet 1603.

Further, the cross section of the prism sheet 1603 is not limited to asawtooth-shape illustrated in FIG. 27. The prism sheet 1603 may have ashape with which light from the light guide plate 1605 can beconcentrated on the liquid crystal panel 1601 side.

The circuit substrate 1608 is provided with a circuit which generatesvarious kinds of signals input to the liquid crystal panel 1601, acircuit which processes the signals, or the like. In FIG. 27, thecircuit substrate 1608 and the liquid crystal panel 1601 are connectedto each other through an FPC (flexible printed circuit) 1609. Note thatthe above-described circuits may be connected to the liquid crystalpanel 1601 by a COG (chip on glass) method, or part of the circuits maybe connected to the liquid crystal panel 1601 by a COF (chip on film)method.

FIG. 27 illustrates an example in which the circuit substrate 1608 isprovided with a controlling circuit which controls driving of the lightsource 1607 and the controlling circuit and the light source 1607 areconnected to each other via the FPC 1610. Note that the above-describedcontrolling circuits may be formed over the liquid crystal panel 1601.In that case, the liquid crystal panel 1601 and the light source 1607are connected to each other through an FPC or the like.

Note that although FIG. 27 illustrates an edge-light type light sourcewhere the light source 1607 is provided on the edge of the liquidcrystal panel 1601, a direct type light source where the light sources1607 are provided directly below the liquid crystal panel 1601 may beused.

This embodiment can be implemented in combination with any of the aboveembodiments as appropriate.

Embodiment 9

In this embodiment, a structure of a light-emitting device including thethin film transistor according to one embodiment of the presentinvention for a pixel is described. In this embodiment, across-sectional structure of a pixel in the case where a transistor fordriving a light-emitting element is n-channel type is described withreference to FIGS. 26A to 26C. Note that, although FIGS. 26A to 26Cshows the case where a first electrode is a cathode and a secondelectrode is an anode, the first electrode may be an anode and thesecond electrode may be a cathode as well.

A cross-sectional view of a pixel in the case where a transistor 6031 isn-channel type, and light emitted from a light-emitting element 6033 isextracted from a first electrode 6034 side is illustrated in FIG. 26A.The transistor 6031 is covered with an insulating film 6037, and overthe insulating film 6037, a bank 6038 having an opening is formed. Inthe opening of the bank 6038, the first electrode 6034 is partiallyexposed, and the first electrode 6034, an electroluminescent layer 6035,and a second electrode 6036 are sequentially stacked in the opening.

The first electrode 6034 is formed of a material or to a thickness totransmit light, and can be formed of a material having a low workfunction of a metal, an alloy, an electrically conductive compound, amixture thereof, or the like. Specifically, an alkaline metal such as Lior Cs, an alkaline earth metal such as Mg, Ca, or Sr, an alloycontaining such metals (e.g., Mg:Ag, Al:Li, or Mg:In), a compound ofsuch materials (e.g., calcium fluoride or calcium nitride), or arare-earth metal such as Yb or Er can be used. Further, in the casewhere an electron injection layer is provided, another conductive layersuch as an aluminum layer may be used as well. Then, the first electrode6034 is formed to a thickness to transmit light (preferably, about 5 nmto 30 nm). Furthermore, the sheet resistance of the first electrode 6034may be suppressed by formation of a light-transmitting conductive layerof a light-transmitting oxide conductive material so as to be in contactwith and over or under the above-described conductive layer with athickness to transmit light. Alternatively, the first electrode 6034 maybe formed of only a conductive layer of another light-transmitting oxideconductive material such as indium tin oxide (ITO), zinc oxide (ZnO),indium zinc oxide (IZO), or gallium-doped zinc oxide (GZO). Furthermore,a mixture in which zinc oxide (ZnO) is mixed at 2% to 20% in indium tinoxide including ITO and silicon oxide (hereinafter referred to as ITSO)or in indium oxide including silicon oxide may be used as well. In thecase of using the light-transmitting oxide conductive material, it ispreferable to provide an electron injection layer in theelectroluminescent layer 6035.

The second electrode 6036 is formed of a material and to a thickness toreflect or shield light, and can be formed of a material suitable forbeing used as an anode. For example, a single-layer film including oneor more of titanium nitride, zirconium nitride, titanium, tungsten,nickel, platinum, chromium, silver, aluminum, and the like, a stackedlayer of a titanium nitride film and a film including aluminum as a maincomponent, a three-layer structure of a titanium nitride film, a filmincluding aluminum as a main component, and a titanium nitride film, orthe like can be used for the second electrode 6036.

The electroluminescent layer 6035 is formed using a single layer or aplurality of layers. When the electroluminescent layer 6035 is formedwith a plurality of layers, these layers can be classified into a holeinjection layer, a hole transport layer, a light-emitting layer, anelectron transport layer, an electron injection layer, and the like inview of the carrier transporting property. In the case where theelectroluminescent layer 6035 includes at least one of the holeinjection layer, the hole transport layer, the electron transport layer,and the electron injection layer in addition to the light-emittinglayer, the electron injection layer, the electron transport layer, thelight-emitting layer, the hole transport layer, and the hole injectionlayer are sequentially stacked over the first electrode 6034 in thisorder. Note that the boundary between each layer is not necessarilyclear, and there may be a case where the boundary is unclear since amaterial for forming each layer is mixed with each other. Each layer canbe formed with an organic material or an inorganic material. As theorganic material, any of a high molecular compound, a medium molecularcompound, and a low molecular compound can be used. Note that the mediummolecular weight material corresponds to a low polymer in which thenumber of repetitions of a structural unit (the degree ofpolymerization) is about 2 to 20. A distinction between a hole injectionlayer and a hole transport layer is not always distinct, which is thesame as in the sense that a hole transporting property (hole mobility)is an especially important characteristic. A layer being in contact withthe anode is referred to as a hole injection layer and a layer being incontact with the hole injection layer is referred to as a hole transportlayer for convenience. The same is also true for the electron transportlayer and the electron injection layer; a layer being in contact withthe cathode is referred to as an electron injection layer and a layerbeing in contact with the electron injection layer is referred to as anelectron transport layer. In some cases, the light-emitting layer alsofunctions as the electron transport layer, and it is therefore referredto as a light-emitting electron transport layer, too.

In the case of the pixel shown in FIG. 26A, light emitted from thelight-emitting element 6033 can be extracted from the first electrode6034 side as shown by a hollow arrow.

Next, a cross-sectional view of a pixel in the case where a transistor6041 is n-channel type, and light emitted from a light-emitting element6043 is extracted from a second electrode 6046 side is illustrated inFIG. 26B. The transistor 6041 is covered with an insulating film 6047,and over the insulating film 6047, a bank 6048 having an opening isformed. In the opening of the bank 6048, a first electrode 6044 ispartially exposed, and the first electrode 6044, an electroluminescentlayer 6045, and the second electrode 6046 are sequentially stacked inthe opening.

The first electrode 6044 is formed of a material and to a thickness toreflect or shield light, and can be formed of a material having a lowwork function of a metal, an alloy, an electrically conductive compound,a mixture thereof, or the like. Specifically, an alkaline metal such asLi or Cs, an alkaline earth metal such as Mg, Ca, or Sr, an alloycontaining such metals (e.g., Mg:Ag, Al:Li, or Mg:In), a compound ofsuch materials (e.g., calcium fluoride or calcium nitride), or arare-earth metal such as Yb or Er can be used. Further, in the casewhere an electron injection layer is provided, another conductive layersuch as an aluminum layer may be used as well.

The second electrode 6046 is formed of a material or to a thickness totransmit light, and formed of a material suitable for being used as ananode. For example, another light-transmitting oxide conductive materialsuch as indium tin oxide (ITO), zinc oxide (ZnO), indium zinc oxide(IZO), or gallium-doped zinc oxide (GZO) can be used for the secondelectrode 6046. Further, a mixture in which zinc oxide (ZnO) is mixed at2% to 20% in indium tin oxide including ITO and silicon oxide(hereinafter referred to as ITSO) or in indium oxide including siliconoxide may be used as well for the second electrode 6046. Furthermore, asingle-layer film including one or more of titanium nitride, zirconiumnitride, titanium, tungsten, nickel, platinum, chromium, silver,aluminum, and the like, a stacked layer of a titanium nitride film and afilm including aluminum as a main component, a three-layer structure ofa titanium nitride film, a film including aluminum as a main component,and a titanium nitride film, or the like can be used for the secondelectrode 6046. However, in the case of using a material other than thelight-transmitting oxide conductive material, the second electrode 6046is formed to a thickness to transmit light (preferably, about 5 nm to 30nm).

The electroluminescent layer 6045 can be formed in a manner similar tothe electroluminescent layer 6035 of FIG. 26A.

In the case of the pixel shown in FIG. 26B, light emitted from thelight-emitting element 6043 can be extracted from the second electrode6046 as shown by a hollow arrow.

Next, a cross-sectional view of a pixel in the case where a transistor6051 is n-channel type, and light emitted from a light-emitting element6053 is extracted from a first electrode 6054 side and a secondelectrode 6056 side is illustrated in FIG. 26C.

The transistor 6051 is covered with an insulating film 6057, and overthe insulating film 6057, a bank 6058 having an opening is formed. Inthe opening of the bank 6058, the first electrode 6054 is partiallyexposed, and the first electrode 6054, an electroluminescent layer 6055,and the second electrode 6056 are sequentially stacked in the opening.

The first electrode 6054 can be formed in a manner similar to that ofthe first electrode 6034 of FIG. 26A. The second electrode 6056 can beformed in a manner similar to the second electrode 6046 of FIG. 26B. Theelectroluminescent layer 6055 can be formed in the same manner as theelectroluminescent layer 6035 in FIG. 26A.

In the case of the pixel shown in FIG. 26C, light emitted from thelight-emitting element 6053 can be extracted from the first electrode6054 side and the second electrode 6056 side as shown by hollow arrows.

This embodiment can be implemented in combination with any of the aboveembodiments as appropriate.

Example 1

By using a semiconductor display device according to one embodiment ofthe present invention, an electronic device with high-speed operationcan be provided. In addition, by using a semiconductor display deviceaccording to one embodiment of the present invention, an electronicdevice capable of displaying an image with high contrast and visibilitycan be provided.

Moreover, with the semiconductor device of the present invention, theheat treatment temperature in the manufacturing process can besuppressed; therefore, a highly reliable thin film transistor withexcellent characteristics can be formed even when the transistor isformed over a substrate formed using a flexible synthetic resin of whichheat resistance is lower than that of glass, such as plastic.Accordingly, with the use of the manufacturing method according to anembodiment of the present invention, a highly reliable, lightweight, andflexible semiconductor device with low power consumption can beprovided. Examples of a plastic substrate include polyester typified bypolyethylene terephthalate (PET), polyethersulfone (PES), polyethylenenaphthalate (PEN), polycarbonate (PC), polyetheretherketone (PEEK),polysulfone (PSF), polyetherimide (PEI), polyarylate (PAR), polybutyleneterephthalate (PBT), polyimide, acrylonitrile-butadiene-styrene resin,polyvinyl chloride, polypropylene, polyvinyl acetate, acrylic resin, andthe like.

Semiconductor devices according to an embodiment of the presentinvention can be used for display devices, laptops, or image reproducingdevices provided with recording media (typically, devices whichreproduce the content of recording media such as digital versatile discs(DVDs) and have displays for displaying the reproduced images). Further,the electronic devices including the semiconductor device according toan embodiment of the present invention include mobile phones, portablegame machines, portable information terminals, e-book readers, camerassuch as video cameras or digital still cameras, goggle-type displays(head mounted displays), navigation systems, audio reproducing devices(for example, car audio systems or digital audio players), copyingmachines, facsimiles, printers, versatile printers, automated tellermachines (ATMs), vending machines, and the like. Specific examples ofsuch electronic devices are shown in FIGS. 28A to 28E.

FIG. 28A illustrates an e-book reader including a housing 7001, adisplay portion 7002, and the like. The semiconductor display deviceaccording to an embodiment of the present invention can be used for thedisplay portion 7002. By including the semiconductor display deviceaccording to one embodiment of the present invention in the displayportion 7002, an e-book reader capable of displaying an image with highcontrast and visibility can be provided. The semiconductor deviceaccording to one embodiment of the present invention can also be usedfor an integrated circuit for controlling the driving of the e-bookreader. By using the semiconductor device according to one embodiment ofthe present invention for the integrated circuit for controlling thedriving of the e-book reader, an e-book reader capable of high-speedoperation can be provided. Moreover, with the use of a flexiblesubstrate, the semiconductor device and the semiconductor display devicecan have flexibility. Thus, a flexible, lightweight, and easy-to-usee-book reader can be provided.

FIG. 28B illustrates a display device that includes a housing 7011, adisplay portion 7012, a support 7013, and the like. The semiconductordisplay device according to an embodiment of the present invention canbe used for the display portion 7012. By including the semiconductordisplay device according to one embodiment of the present invention inthe display portion 7012, a display device capable of displaying animage with high contrast and visibility can be provided. Thesemiconductor device according to one embodiment of the presentinvention can also be used for an integrated circuit for controlling thedriving of the display device. By using the semiconductor deviceaccording to one embodiment of the present invention for the integratedcircuit for controlling the driving of the display device, a displaydevice capable of high-speed operation can be provided. Note thatexamples of the display device include all the information displaydevices used for personal computers, TV broadcast reception,advertisement display, or the like.

FIG. 28C illustrates a display device including a housing 7021, adisplay portion 7022, and the like. The semiconductor display deviceaccording to an embodiment of the present invention can be used for thedisplay portion 7022. By including the semiconductor display deviceaccording to one embodiment of the present invention in the displayportion 7022, a display device capable of displaying an image with highcontrast and visibility can be provided. The semiconductor deviceaccording to one embodiment of the present invention can also be usedfor an integrated circuit for controlling the driving of the displaydevice. By using the semiconductor device according to one embodiment ofthe present invention for the integrated circuit for controlling thedriving of the display device, a display device capable of high-speedoperation can be provided. Moreover, with the use of a flexiblesubstrate, the semiconductor device and the semiconductor display devicecan have flexibility. Thus, a flexible, lightweight, and easy-to-usedisplay device can be provided. Accordingly, as illustrated in FIG. 28C,a display device can be used while being fixed to fabric or the like,and an application range of the display device is dramatically widened.

FIG. 28D illustrates a portable game machine including a housing 7031, ahousing 7032, a display portion 7033, a display portion 7034, amicrophone 7035, speakers 7036, operation keys 7037, a stylus 7038, andthe like. The semiconductor display device according to an embodiment ofthe present invention can be used for the display portion 7033 and thedisplay portion 7034. By including the semiconductor display deviceaccording to one embodiment of the present invention in the displayportion 7033 and the display portion 7034, a portable game machinecapable of displaying an image with high contrast and visibility can beprovided. The semiconductor device according to one embodiment of thepresent invention can also be used for an integrated circuit forcontrolling the driving of the portable game machine. By using thesemiconductor device according to one embodiment of the presentinvention for the integrated circuit for controlling the driving of theportable game machine, a portable game machine capable of high-speedoperation can be provided. Although the portable game machineillustrated in FIG. 28D has the two display portions 7033 and 7034, thenumber of display portions included in the portable game machines is notlimited thereto.

FIG. 28E illustrates a mobile phone which includes a housing 7041, adisplay portion 7042, an audio input portion 7043, an audio outputportion 7044, operation keys 7045, a light-receiving portion 7046, andthe like. Light received in the light-receiving portion 7046 isconverted into electrical signals, whereby external images can beloaded. The semiconductor display device according to an embodiment ofthe present invention can be used for the display portion 7042. Byincluding the semiconductor display device according to one embodimentof the present invention in the display portion 7042, a mobile phonecapable of displaying an image with high contrast and visibility can beprovided. The semiconductor device according to one embodiment of thepresent invention can also be used for an integrated circuit forcontrolling the driving of the mobile phone. By using the semiconductordevice according to one embodiment of the present invention for theintegrated circuit for controlling the driving of the mobile phone, amobile phone capable of high-speed operation can be provided.

Example 1 can be implemented in combination with any of the aboveembodiments as appropriate.

This application is based on Japanese Patent Application serial no.2009-235570 filed with Japan Patent Office on Oct. 9, 2009, the entirecontents of which are hereby incorporated by reference.

EXPLANATION OF REFERENCE

10: pulse output circuit, 11: wiring, 12: wiring, 13: wiring, 14:wiring, 15: wiring, 21: input terminal, 22: input terminal, 23: inputterminal, 24: input terminal, 25: input terminal, 26: output terminal,27: output terminal, 31: transistor, 32: transistor, 33: transistor, 34:transistor, 35: transistor, 36: transistor, 37: transistor, 38:transistor, 39: transistor, 40: transistor, 41: transistor, 42:transistor, 43: transistor, 51: power supply line, 52: power supplyline, 53: power supply line, 201: thin film transistor, 202: substrate,203: gate electrode, 204: gate insulating film, 205: oxide semiconductorfilm, 206: source electrode, 207: drain electrode, 208: oxide insulatingfilm, 209: conductive film, 210: insulating film, 211: thin filmtransistor, 212: substrate, 213: gate electrode, 214: gate insulatingfilm, 215: oxide semiconductor film, 216: source electrode, 217: drainelectrode, 218: oxide insulating film, 219: conductive film, 220:insulating film, 221: thin film transistor, 222: substrate, 223: gateelectrode, 224: gate insulating film, 225: oxide semiconductor film,226: source electrode, 227: drain electrode, 228: oxide insulating film,229: conductive film, 230: insulating film, 231: channel protectivefilm, 250: composite layer, 251: metal oxide film, 260: composite layer,261: metal oxide film, 270: composite layer, 271: metal oxide film, 400:substrate, 401: gate electrode, 402: gate insulating film, 403: oxidesemiconductor film, 404: oxide semiconductor film, 405: oxidesemiconductor film, 406: conductive film, 408: capacitor wiring, 409:oxide semiconductor film, 411: oxide insulating film, 412: oxidesemiconductor film, 413: thin film transistor, 414: pixel electrode,415: transparent conductive film, 416: transparent conductive film, 420:terminal, 421: terminal, 430: composite layer, 431: metal oxide film,700: pixel portion, 701: signal line driver circuit, 702: scan linedriver circuit, 703: pixel, 704: transistor, 705: display element, 706:storage capacitor, 707: signal line, 708: scan line, 710: pixelelectrode, 711: counter electrode, 712: microcapsule, 713: drainelectrode, 714: resin, 1401: thin film transistor, 1402: gate electrode,1403: gate insulating film, 1404: oxide semiconductor film, 1406:conductive film, 1407: oxide insulating film, 1408: insulating film,1410: pixel electrode, 1411: alignment film, 1413: counter electrode,1414: alignment film, 1415: liquid crystal, 1416: sealant, 1417: spacer,1420: composite layer, 1421: metal oxide film, 1601: liquid crystalpanel, 1602: diffusing plate, 1603: prism sheet, 1604: diffusing plate,1605: light guide plate, 1606: reflection plate, 1607: light source,1608: circuit substrate, 1609: FPC, 1610: FPC, 407 a: source electrode,407 b: drain electrode, 5300: substrate, 5301: pixel portion, 5302: scanline driver circuit, 5303: scan line driver circuit, 5304: signal linedriver circuit, 5305: timing control circuit, 5601: shift register,5602: sampling circuit, 5603: transistor, 5604: wiring, 5605: wiring,6031: transistor, 6033: light-emitting element, 6034: electrode, 6035:electroluminescent layer, 6036: electrode, 6037: insulating film, 6038:bank, 6041: transistor, 6043: light-emitting element, 6044: electrode,6045: electroluminescent layer, 6046: electrode, 6047: insulating film,6048: bank, 6051: transistor, 6053: light-emitting element, 6054:electrode, 6055: electroluminescent layer, 6056: electrode, 6057:insulating film, 6058: bank, 7001: housing, 7002: display portion, 7011:housing, 7012: display portion, 7013: support, 7021: housing, 7022:display portion, 7031: housing, 7032: housing, 7033: display portion,7034: display portion, 7035: microphone, 7036: speaker, 7037: operationkey, 7038: stylus, 7041: housing, 7042: display portion, 7043: audioinput portion, 7044: audio output portion, 7045: operation key, 7046:light-receiving portion.

1. A semiconductor device comprising: a gate electrode over aninsulating surface; a gate insulating film over the gate electrode; anoxide semiconductor film over the gate insulating film, the oxidesemiconductor film overlapping with the gate electrode; a pair of metaloxide films over the oxide semiconductor film; and a source electrodeand a drain electrode over the pair of metal oxide films, wherein theoxide semiconductor film comprises, on the pair of metal oxide filmssides, a region where a concentration of one or a plurality of metals inthe oxide semiconductor film is higher than a concentration of one or aplurality of metals in other regions of the oxide semiconductor film,and wherein the pair of metal oxide films are formed by oxidation of ametal in the source electrode and the drain electrode.
 2. Thesemiconductor device according to claim 1, wherein the metal in thesource electrode and the drain electrode is titanium, tungsten, ormolybdenum.
 3. The semiconductor device according to claim 1, wherein athickness of the region of the oxide semiconductor film is more than orequal to 2 nm and less than or equal to 10 nm.
 4. The semiconductordevice according to claim 1, wherein a thickness of each of the pair ofmetal oxide films is more than or equal to 2 nm and less than or equalto 10 nm.
 5. The semiconductor device according to claim 1, furthercomprising an insulating film over the source electrode and the drainelectrode, wherein the insulating film is in direct contact with theoxide semiconductor film.
 6. A semiconductor device comprising: a gateelectrode over an insulating surface; a gate insulating film over thegate electrode; an oxide semiconductor film including indium, gallium,and zinc over the gate insulating film, the oxide semiconductor filmoverlapping with the gate electrode; a pair of metal oxide films overthe oxide semiconductor film; and a source electrode and a drainelectrode over the pair of metal oxide films, wherein the oxidesemiconductor film comprises, on the pair of metal oxide films sides, aregion where a concentration of one or a plurality of indium, gallium,and zinc is higher than a concentration of one or a plurality of indium,gallium, and zinc in other regions of the oxide semiconductor film, andwherein the pair of metal oxide films are formed by oxidation of a metalin the source electrode and the drain electrode.
 7. The semiconductordevice according to claim 6, wherein the metal in the source electrodeand the drain electrode is titanium, tungsten, or molybdenum.
 8. Thesemiconductor device according to claim 6, wherein a thickness of theregion of the oxide semiconductor film is more than or equal to 2 nm andless than or equal to 10 nm.
 9. The semiconductor device according toclaim 6, wherein a thickness of each of the pair of metal oxide films ismore than or equal to 2 nm and less than or equal to 10 nm.
 10. Thesemiconductor device according to claim 6, further comprising aninsulating film over the source electrode and the drain electrode,wherein the insulating film is in direct contact with the oxidesemiconductor film.
 11. A semiconductor device comprising: a gateelectrode over a substrate; a gate insulating film over the gateelectrode; an oxide semiconductor film comprising a first metal over thegate insulating film, the oxide semiconductor film overlapping with thegate electrode; a first metal oxide film over the oxide semiconductorfilm; a second metal oxide film over the oxide semiconductor film; asource electrode over the first metal oxide film, the source electrodebeing in direct contact with the gate insulating film; and a drainelectrode over the second metal oxide film, the drain electrode being indirect contact with the gate insulating film; wherein each of the firstmetal oxide film, the second metal oxide film, the source electrode andthe drain electrode comprises a second metal, wherein a first region ofthe oxide semiconductor film is adjacent to the first metal oxide film,wherein a second region of the oxide semiconductor film is adjacent tothe second metal oxide film, wherein a third region of the oxidesemiconductor film is adjacent to the gate insulating film, and whereina concentration of the first metal in the first region and the secondregion is higher than the concentration of the first metal in the thirdregion.
 12. The semiconductor device according to claim 11, wherein thesecond metal is titanium, tungsten, or molybdenum.
 13. The semiconductordevice according to claim 11, wherein a thickness of each of the firstregion and the second region is more than or equal to 2 nm and less thanor equal to 10 nm.
 14. The semiconductor device according to claim 11,wherein a thickness of each of the first metal oxide film and the secondmetal oxide film is more than or equal to 2 nm and less than or equal to10 nm.
 15. The semiconductor device according to claim 11, furthercomprising an insulating film over the source electrode and the drainelectrode, wherein the insulating film is in direct contact with theoxide semiconductor film.
 16. A method for manufacturing a semiconductordevice, the method comprising the steps of: forming a gate electrodeover a substrate; forming a gate insulating film over the gateelectrode; forming an oxide semiconductor film over the gate insulatingfilm; forming a conductive film over the oxide semiconductor film;conducting a heating treatment for the oxide semiconductor film to forma metal oxide film between the oxide semiconductor film and theconductive film after the forming of the conductive film; and etchingthe conductive film to form a source electrode and a drain electrode.17. The method according to claim 16, wherein the oxide semiconductorfilm is formed by a sputtering method while heating the substrate at atemperature of more than or equal to 400° C. and less than or equal to700° C.